Some promising leads will pan out, but also some innocent people may be swept up in the dragnet.
One American who knows all about that is Richard Jewell, who in 1996 was falsely accused of a terrorist act - setting off a bomb at the Atlanta Olympics.
In a very public investigation by the FBI and the Atlanta police, Jewell was branded "the chief suspect in the bombing."
Correpsondent Mike Wallace got to know Richard Jewell back in 1996, but it wasn't until recently that he realized how deeply affected Jewell was by his ordeal and why it's been so difficult for him to recover from what happened then.
It all began when the bomb exploded in Atlanta's Centennial Olympic Park just after 1 a.m.on July 27, 1996. Richard Jewell was working as a private security guard there and helped escort many of the spectators to safety. He was called a hero. Then everything changed.
Jewell says a headline in the Atlanta Journal Constitution that afternoon "pretty much started the whirlwind." The headline read: FBI suspects hero guard may have planted bomb.
It was read to the world verbatim as breaking news on CNN and the AP picked it up, sending it to news organizations around the world.
The story leaked to the media was that Jewell wasn't a hero at all, but was himself the bomber who perversely sought publicity for saving people from the explosion.
"Everybody then assumed that this bizarre character, as he was being portrayed, had decided that this was gonna be his 15 minutes of fame, that he was going to set up this situation where he would literally bomb a park and then claim to be a hero," says Lin Wood, Jewell's lawyer.
The FBI put Jewell under round-the-clock surveillance and conducted a very public search of his apartment. All of it was broadcast on live television. It was not until many weeks later that the FBI finally acknowledged Jewell was not a suspect in the bombing.
In five years, the FBI has never apologized for the leak. "Nobody's ever called me, written me a letter, sent me an Email, called any of my attorneys," Jewell tells Wallace.
Jewell in fact saved over 100 innocent peoples' lives that night. And no one has ever bothered to even say thanks - not the city of Atlanta, not the state of Georgia, not the Olympic Committee in Atlanta, not the International Committee.
"He's so tainted that even when he was exonerated, no one still wanted to really be identified with him," lawyer Wood says.
Even Wood once thought Jewell was guilty. "I actually believed what I saw on television and what I read in the newspapers and I thought that the FBI had their man and the man was Richard Jewell," Wood says.
When Wallace first met Jewell a few weeks after the bombing, Jewell was till a suspect. Wallace says he though Jewell might indeed be the bomber until Jewell described what had happened that night when he noticed a suspicious-looking package under a park bench and pointed it out to a federal ATF agent.
"He ( the ATF agent) was laying flat on his stomach and he was undoing the top of the bag with his hand and he was doing his flashlight like this and all of a sudden, he just froze," Jewell told Wallace. "What really made me think this is, 'oh, oh, this is bad', is there was like a little line in training that they taught you that would instill in you, and it was, 'if you see an ATF agent running, you better be in front of him.'"
But instead of running, Jewell stayed within 10 yards of where the package was. "We were just concerned with getting the people as far away as we could, as quickly as we could," he says.
That summer, Jewell was besieged by the media wherever he went. And the FBI kept following him, too. When he went out for a drive, they were always there. Not just one agent in a Jeep, but a whole FBI convoy - a total of five or six FBI vehicles- which chased him on and off the Interstate The FBI also searched the home of Jewell's mother, Bobbi.
Out of her apartment they took Jewell's guns - he hunts deer - all the Tupperware and a collection of 22 Walt Disney tapes.
Through it all, TV cameras rolled. "I felt like it was a movie production," Mrs. Jewell says. "Every night, there must have been 20 satellite trucks in our parking lot."
As the weeks went by, the torment by the media turned to ridicule.
Jay Leno called Jewell a "Una-doofus." A federal agent was quoted as saying Jewell was a "Una-bubba." His mother was called the "Una-Momma."
In late October of 1996, the U.S. Attorney finally wrote a letter saying Jewell was not considered a target of the federal criminal investigation - a statement delivered to one of Jewell's lawyers at an out-of-the-way coffee shop, far away from the TV cameras. And unlike the very public FBI investigation of Jewell, there was no FBI press conference. Jewell and his attorney, Lin Wood, say the FBI was simply too embarrassed.
"We know a lot more than we knew at the time, when we sat down and talked with you initially," Wood tells Wallace. "We know, for example, that the FBI was interested in Richard, but had really not decided whether Richard Jewell was a possible suspect or a potentially valuable witness. But before they could execute their plan, the banner headline gets published, and now all of a sudden, the FBI's got to come to grips with Richard Jewell in a public investigation, and that changed, I think, the whole approach that the FBI took."
Jewell had planned to sue the FBI, but eventually decided it would be futile. He
"The plaintiff has consistently complained that he should never have been a suspect of the FBI in the bombing," he says. "But he has never sued the FBI. Instead he has asserted that the Journal Constitution is to blame for all his troubles by being the first to accurately report that he had in fact become the investigation's focus."
Wood responds, " Look, call him a suspect. Put it in your newspaper. You will not be sued for calling Richard Jewell a suspect. You will be sued when you publish in your newspaper that he's a villain, a bad man. He fits the profile of the lone bomber of the park. He sought publicity. And he's like Wayne Williams, a convicted murderer of children. Put that in your newspaper and you will be sued. That's what the Atlanta Journal and Constitution said about this man."
For the last five years, Jewell has been trying to put his life back together. He did manage to find a job as a police officer in a small community. He lost 65 pounds and got married last September - a private affair with no TV cameras. Jewell is still just as angered at what the media did to him as he was back in 1996.
Wood says suspicion remains. "You'd be surprised," he says, "to know how many people come up to me and will still nudge me and say, 'Hey, so tell me, did he really do it ?' That question is still asked."
When he goes to a grocery store, Jewell says, people still stare, point and whisper his name.
"Driving home from work," Jewell says, "you're always looking in your rearview mirror. If you see a car there, the same car for a long time, you turn off, even though it's not your turn, to see if that car follows you. You drive different, drive a different way home every night.
"When you come out to get in your, your car in the morning, you walk around it and look under it. And you look around the parking lot to see if somebody's sitting in a car."
While it turns out Jewell was a hero, not the bomber, he's never been treated like a hero.
"I don't know what a hero's treated like," he says, "but my mother and I have never been treated like that."
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