Why would they stop investigating on Arthur Leigh Allen?

Well, that's precisely LAW for you. Even if you understand well enough that certain person is guilty, you have to prove it by evidence.

I remember this scene from the movie's last 15 minutes, where Jake Gylenhaal's character and Mark Ruffalo's character meet in the early hours. Gylenhaal tells Ruffalo that Arthur and Darlene lived door to door once. This was good evidence, but still, and I precisely remember the dialogues, since I have watched that movie 3-4 times myself.

Ruffalo (Detective) : Unbelievable ! But the fingerprints, the hand writings...
Gylenhaal (Cartoonist) : I am not asking you as a Police Officer.
Ruffalo : But I AM a Police Officer.

This conversation pretty much sums it up. The evidence they had against Lee was circumstantial. So they needed some concrete evidence like hand writing match or fingerprint match to prove him guilty. Otherwise they needed something else to build a strong case against Lee.

During the last scene the person who was shot by Lee on 4th July identifies him and this indeed was a string enough evidence. If you remember, they show it just before the credits that Lee was summoned following his identification but he died a natural death instead.

From what i remember Arthur was never ruled out, he was on the top of list of suspects. He was never arrested because one way or another something came in between whenever they decide to arrest him. Maybe its the difficulty in getting pass through the hurdles of law to make an actual arrest. they never had anything concrete. I am in no way an expert in laws of USA, but from movies i have seen i know its very tight, for example: i have seen frustrating (and as well as irritating) situations in which we know the bad guy is behind everything, we can trap him by searching his home or tracking his phone but law says you cant do that.

i think the same happened in Allen's case, the final thing, the concrete thing to make the arrest was not there. that added with confusions that came in between like

1. other crimes getting named under zodiac.
2. people's claiming and giving false information (there is a specific scene in movie showing this)

restricted the police in arresting him. That along with lack of coordination among different police departments, i remember the crimes happened in places under different police departments (Napa valley, Vallejo etc )

also in the last scrolls it is said that the DNA of Allen didn't match the partial DNA obtained from the letters sent to San Francisco chronicle.

I think Arthur Leigh Allen is the most likely suspect. There was heaps of circumstantial evidence but no hard proof which would have made charges difficult. Whoever the Zodiac was, there would have been individuals with things in common with him. If it wasn't Allen he was in the top 5 who resembled him the most.

Here is a way of looking at it. I'm making up the figures but I will show a possible approach.

Lets assume Zodiac liked in the Vallejo area, lets say there were 100,000 adult males there at that time. The probability that Allen was the Zodiac is 1 in 100,000. Lets say 1 in 5 of these were white, stocky and the age group that both Allen and Zodiac were. Its now 1 in 20,000. Allen and Zodiac had the same shoe size. Say 1 in 2 of this group have this size so its now 1 in 10,000.

Allen and Zodiac had the same unusual typewriter. Lets say 1 in 100 have it. Its now 1 in 1000. Allen had no alibis for the time of the attacks. Lets say 1 in 10 have no alibi. Its now 1 in 100.

Zodiac quotes from a book 'the most dangerous game' that Allen has read. Say 1 in 10 have read the book (probably less). Its now one in ten.

And Allen has talked about attaching a flashflight to a gun, hunting people, loosening lug nuts on cars, using the zodiac symbol, mucks around with codes, deliberately mispells words, has plans/equipment for making bombs, military training etc etc. The probability that Allen and Zodiac are the same is getting rather high.

The lack of DNA evidence means nothing. It is likely there is no Zodiac DNA on any envelope. A clever Zodiac could disguise his handwriting and not put saliva on envelopes.

The best evidence against Allen is that witness Bryan Hartnell, who comes across as quite intelligent, believes its Allen because the voice is the same. Allen had quite a distinctive voice.

Also Allen repeats himself when he denied he is the Zodiac, which is sign of deception.

From the silence of the lambs:

Hannibal Lecter: First principles, Clarice. Simplicity. Read Marcus Aurelius. Of each particular thing ask: what is it in itself? What is its nature? What does he do, this man you seek?

Clarice Starling: He kills women...

Hannibal Lecter: No. That is incidental. What is the first and principal thing he does? What needs does he serve by killing?

Clarice Starling: Anger, um, social acceptance, and, huh, sexual frustrations, sir...

Hannibal Lecter: No! He covets. That is his nature. And how do we begin to covet, Clarice? Do we seek out things to covet? Make an effort to answer now.

Clarice Starling: No. We just...

Hannibal Lecter: No. We begin by coveting what we see every day. Don't you feel eyes moving over your body, Clarice? And don't your eyes seek out the things you want?

Arthur Leigh Allen lived right near Darlenes job. He probably saw her all the time and became obsessed with her. She was his first. Its simple. Arthur Leigh Allen did it. The other evidence - or lack of - doesnt matter . The rest of the evidence against him DOES. Its over. Zodiac is dead and rotting in hell.

"Proof" -

1. zodiac watch - unless it was unique, or noted as part of the crimes, that's not really proof of anything.

2. Matching the description - important, but not really definitive without other evidence

3. Arrested for molesting children - has really no relation to the murders, at all, and would never be allowed to be presented in court unless murder victims were children who were molested

4. Information the colleague gave helps to keep him as a suspect, but does not link him to specific crimes

5. Living a few yards away, see #4,

6. Cousin remembered, but is that because he's the criminal, or because of #5 - he lives in the area?

All the proof is very circumstantial. Certainly enough to make them strongly suspect him, but not actual evidence that would get him convicted, and most of it not actual proof of anything specific. They "let" him continue to roam around because they lacked the evidence to the actual crimes to do otherwise.

It wasn't the handwriting alone. The documents examiner for the San Francisco Police Department said that he was given "a banana box full of Allens handwriting and it wasn't even close". That means the probability of Allen changing his handwriting that much was remote, as the examiner would have noticed the examples over time changing. In 2002, DNA analysis, again, "wasn't even close", along with another suspect that they checked. No handwriting match, no DNA match does not mean there isn't enough evidence, it means he didn't do it !!!! Eyewitness testimony is the LEAST reliable form of evidence there is, it has been proven as such many, many times. As far as the ton of circumstantial evidence, or as I like to call it, "coincidences", I am sure you all have done extensive researching online, as have I, and you can come up with a million coincidences a day with names, dates, etc. overlapping to a degree that I have actually changed my belief of "circumstantial" evidence pertaining to everything, including people in my own life, lol. So, in answer to your question of "how could they let him run around with so much proof?" The answer is - they have absolutely no proof. Not one bit of "proof" ( which you are confusing with "evidence", by the way ) The finding that his drivers license had an issue date that coincided with the postmark date of a Zodiac letter is absolutely, positively, 100% proof that that the date his license was issued matches the date of a postmark of a Zodiac letter, lol, nothing else, and that goes for the supposed other "important" dates also. You might even venture to ask yourself a question as to why the hell the issuance date of his drivers license would be important? That kind of evidence gathering is junk, plain and simple. People have watched too many detective shows with the "whodunit" themes, seriously. Take down your family's birthdates and pick some dates of major happenings in the world and see if you can match any up. Do it with SS numbers, car license plates- any numbers you choose and you will find an astonishing number of coincidences. Same goes with names, like them mentioning that the cab dropped the Zodiac off at the "Allen apartments". You would not believe how many times things like this occur. Type random names into google and see how many things pop up, like typing in "Scott Anderson" and watching the results show you how many thousands of people across the country that are named Scott, live on Anderson St., or go to Anderson high school, or are employed by Anderson windows, etc. Put it this way - there are no strange coincidences, there are plenty of common ones! In fact, I challenge anyone to pick numbers, names, etc and see if they can NOT find any coincidences.

you cannot look at things that they have in common, and claim that since so many are common he must be zodiac. You can take any two random people and find dozens of things that they have in common, no matter how rare these things may be in the general population. Hard evidence is all that matters. in the years since the murders DNA has come a long way. If there is no evidence collected from the letters, items he sent, or the crime scene to give away who he was, this case will never be solved unless a credible old man confesses.

So listen. Arthur was ambidextrous and his dominant hand was left. I'm exactly like that. I write with my left, punch with my right, grab with my left, and wipe my butt with my right. This is important: although I write with my left, I also wear my watch on the same wrist. When I was in school, and my dominant left hand got tired, I wrote with the my right, My teachers always had a problem with me when I wrote with my left because the letters was normal but going up from left to right. And when I wrote with my right, the letters were smaller, and going from the left down to the right. Go check a letter by zodiac. The letters go from the left down to the right. Important fact: left handed writing makes the letters smudgy because writing from left to right is designed for right handed people, right handed do not. Zodiac's letters were clean. I do not know much about Arthur, only that he knew how to manipulate people, was a clever guy even though he was an underachiever (exactly like me). Now let me say something even though I would never kill someone. I know the police would never catch me if i did. He did the perfect crime. He found victims he had no relation with, and had no reason to kill them. He studied a lot on how to make those murders and leave no trace. His only mistake was to get angry at that party, but he covered it so wisely because, when they are so close to you, the best way to mess with people is to open suddenly the lights and surprise them. Make them believe it can't be that easy. For the last part, even though I do not believe in astrology or something like that, I was also born on 18-12 like he. When I saw this movie a couple of days ago, I was totally sure it was him because, as far as I could tell from a movie, he was very similar to me, and what he did I would have done the same to cover my tracks. Call me crazy but the same writing method, same birthday, mind of composition to manipulate. It's too much for me. He did the crime exactly the way I would if I were a criminal.

Yeah but there was WAY TOO MANY coincidences. Darlene Ferrin knowing a Leigh, same ammo brand and type of gun, exact same size gloves and boots as Zodiac, misspelling of Christmas, Zodiac brand watch, same rare typewriter, a few different people having supposedly had conversations where he would tape a flashlight to his gun, or picking off the little darling, Hartnell stating the voice of Leigh sounds like the man that stabbed him, was around Riverside in 66, the bloody knives in his car, Mageau picking him out in a random line up(however you are right eyewitness have failed time after time) that's just some of many circumstancial evidence that Allen possibly and probably had something to do with the crimes. It's extremely rare that all these are just a coincidences. Allen likely was the Zodiac.

The DNA evidence just wasn't there. There's no way he could have known DNA would be around 20 30 years later so he wouldn't have had someone else lick the stamps. Now the idea that he may have had a accomplice is interesting and would backup the different composite drawings, different hand writing DNA etc. always have been fascinated with this case, I live in solano county and have visited all the sites around here.