- The search for the cyanide killer is being coordinated from a northwest suburban storefront where law enforcement agents are briefed on the latest evidence, then sent out to track down thousands of leads.

- It was an extremely difficult case to come at because of the randomness of the victims.

It didn't make any sense.

There wasn't anything tying them together.

The person who did this, he didn't know who he was going to kill, - And detectives didn't know if the killer intended to strike again.

Within 24 hours of the poisonings, Illinois Attorney General Tyrone Fahner assembled the largest task force in state history to work the case.

- The number of people involved in the investigation was, was just big.

I mean, a couple hundred people were all said and done, - And law enforcement officials continued what is being called the biggest manhunt in Illinois history.

They looked at known terrorists, known murderers that maybe had been released recently.

Mental patients, anybody that might have a motivation against this product, these companies, these families, - What are they looking for?

He said, we are looking for a madman.

Do you have any kind of a profile on such an individual type?

- No, we do not.

No, no, we do not.

We, we don't know.

It's gotta be somebody who's a little bit nuts.

- The FBI's Behavioral Analysis Unit built a criminal profile of the killer at a time when such methods were fairly new.

While some dismissed the report.

A 34-year-old agent named Roy Lane found its predictions of the killer's Next moves invaluable.

There's a great likelihood the person who was responsible thinks he got away with this and it's very much a high, but over time, that high is going to diminish and he's gonna want to get involved in the investigation so that he can resume that high.

Relentless media coverage ramped up pressure on law enforcement to solve the case.

- There was no bigger story than this back then, and it dominated not only the newsroom and all the people in it, but it also dominated air and every newscast was wall to wall with this.

- The Chicago Police Department set up its own task force to investigate Paula Prince's murder.

Homicide Detective Jimmy Gildea found a tantalizing clue inside her apartment near downtown, - And it was a Walgreens receipt.

The Tylenol was on the receipt and it was timestamped, so we knew where she bought it, and eventually we got the pictures from the Walgreens.

- A security camera captured the moment Prince bought the Tylenol, a unique snapshot in an era before cell phones and surveillance cameras became routine.

- It's chilling.

You're literally watching her buy her own death.

- The photos didn't lead to any viable suspects.

Detectives looked into tips involving chemicals and got a lead from a local bar owner about a customer who claimed to possess cyanide.

- He told us, I got a semi-regular customer to consider here.

He's kind of an odd duck.

He's telling my patrons It's easy to put cyanide in capsules.

You can slip it to people.

You'd never get caught doing something like - That.

The customer was 48-year-old Roger Arnold.

Under questioning Arnold admitted he had purchased cyanide for amateur experiments, but denied being the Tylenol killer.

He agreed to let detectives search his home.

- We find a bunch of guns, a rifle, several pistols, all kinds of beakers and things in the basement.

The Anarchist cookbook.

There are all kinds of books on making bombs.

We find detonators for explosives.

- Detectives didn't find any cyanide, but a look into Arnold's background aroused more suspicion.

He frequented bars near the Walgreens where Paula Prince bought Tylenol.

He worked at a Jewel warehouse, a grocery store chain where two of the tainted bottles had been purchased and he worked with a father of one victim.

- In this guy's case, he checked every box as far as possibly being the guy.

- Arnold was cooperative until the questioning turned to cyanide.

Arnold has refused to take a lie test that can clear him of any connection with the Tylenol murders.

- After his name was leaked to the press, Arnold clammed up and demanded an attorney.

- So it was some glass thrown in front of the tire, that's for sure, - With no direct evidence tying him to the murders.

Arnold was charged with weapons violations and released on $600 bond.

He returned to his job under a cloud of suspicion.

- Roger Arnold was very angry that the finger had been pointed in his direction and he was going to get his revenge, and so he started stalking those bars trying to find, the guy he believed had turned him in as a possible suspect, and one night he saw that person leave the bars.

- So he walks up to me and says, you turned me into the police, didn't you?

So the guy looks at him and says, I'm sorry, boom.

He shoots him right in the chest.

- Gildea questioned Arnold at the police station afterwards.

- I had the driver's license of the guy that he killed, and I showed it to him.

I says, this is who you shot.

You shot the wrong guy.

- The victim, 46-year-old John Stanisha had no connection to the Tylenol case.

He was a computer consultant with three daughters.

- I says, this guy's got kids.

What does she do?

He just collapses.

He just goes berserk, crying and everything else.

He realized that he killed the wrong guy.

- Some people called John Stanisha, the eighth Tylenol victim because as a result of the Tylenol murders, he ended up dead on the sidewalk.

- Arnold was convicted of murdering John Stanishaw and sentenced to 30 years in prison.

He was never charged with the Tylenol poisonings.

- People say, do you think he did it?

The only thing I can say is I can't say that he didn't do it.

It's just these bad taste in your mouth.

We took the best shot at it that we could, you know, we always liked to deliver for the family, you know, but the, we weren't able to, in this case.