BOSTON (WHDH) - Many are left wondering why the Orlando gunman could not have been stopped and the former head of the FBI’s Boston office said it is frustrating when a case slips through the cracks.

Orlando gunman Omar Mateen was once on the FBI’s terror watch list. Rick Deslauriers, who investigated the 2013 Boston Marathon bombings when he was the head of the FBI’s Boston office said it is difficult to track thousands of names on the terror watch list.

“In a lot of ways it’s like a needle in a haystack,” said Deslauriers. “Even when you take a close look at somebody, you never know down the right they might commit some illegal terrorist act. You can’t watch them 24/7, all the time, unless there’s a fairly imminent threat.”

Deslauriers said that there are similarities between Mateen and Tamerlan Tsarnaev, who was also interviewed by the FBI years before the marathon bombings. FBI agents found no links to terrorism at the time and Tsarnaev’s case was closed.

“The FBI does a fantastic job keeping this nation safe but it doesn’t mean every threat posed to this nation is going to be detected in advance by the FBI,” said Deslauriers.

FBI agents in Orlando are being scrutinized to determine if they did enough when investigating Mateen. In Boston, the official report of the FBI’s handling of the Tsarnaev case concluded there were areas where information sharing between federal, local and state law enforcement could have been more effective.

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