Challenges of investigating potential threats

At least one aspect of the Orlando massacre is disturbingly familiar: a troubled man with suspicious behavior who is on the FBI's radar and on a watch list never gets arrested and goes on to take many innocent lives.

Investigators tell us Orlando gunman Omar Mateen claimed allegiance to ISIS and praised the Boston marathon bombers. Three years ago, he drew the attention of the FBI after claiming terrorist ties. The case was closed but he was on watch lists.

We asked Karen Greenberg, the director of Fordham Law School's Center for National Security, to explain. She says the United States has many overlapping lists; a no-fly list, a terrorist watch list and more. She says the largest of these lists has 750,000 people.

Greenberg, whose new book is called "Rogue Justice," says more is definitely not better, and the sheer volume of names diminishes its value.

FBI Director James Comey confirmed his agency's involvement with Mateen. He says the FBI's Miami office investigated Mateen and tried to determine if he was a possible threat.

Critics cite a similarity with the Tsarnaev brothers, where the FBI received a tip from Russian authorities and opened an investigation, but never arrested them. They went on to bomb the Boston Marathon.

Experts estimate more than 900 cases are open nationwide. Comey says the work is very challenging and is like looking for needles in a nationwide haystack.

But Greenberg takes some issue with that assessment. Greenberg says the FBI must figure out what went wrong as soon as possible, and is urging the law enforcement community to hone in on more meaningful lists and assessments of potentially dangerous individuals.