Boston bombing suspect Tamerlan Tsarnaev was on a highly classified terrorist watch list, according to the FBI, but was not monitored by authorities because the list has more than 500,000 names and the bureau could find no evidence that Tsarnaev was a threat.

The terrorist was also on a watch list used by the border and customs services, but was not given any special screening when he left the US for a six month stay in Russia last year. By the time he got back in July, he had been downgraded on the classified terror list.

Trying to get the FBI to divulge what they had on Tamerlan Tsarnaev is turning into an exercise in pulling teeth. The more information that emerges, the clearer it is becoming that there are serious - even fatal - flaws in our terrorist watch system that need to be addressed immediately.

Reuters:

Tsarnaev was flagged on that database when he left the United States for Russia in January 2012 but no alarm was raised, presumably because the FBI had not identified him as a threat after the interview.

When he returned from Russia six months later, he had already been automatically downgraded in the border database because there was no new information that required him to continue to get extra attention. So he did not get secondary inspection on his re-entry at New York's JFK Airport. It was unclear exactly what the procedure was for such a downgrade.

Sean Joyce, deputy director of the FBI, defended the FBI's performance in the Boston bombings at two closed hearings in Congress on Tuesday.

While government agencies declined to publicly discuss how the watch list system handled Tsarnaev, Homeland Security Secretary Janet Napolitano disclosed some details at a separate, open hearing on immigration on Capitol Hill.

"Yes, the system pinged when he was leaving the United States. By the time he returned, all investigations - the matter had been closed," Napolitano told the Senate Judiciary Committee on Tuesday.

Bob Grenier, former chief of the CIA's Counterterrorism Center, said the effectiveness of watch list systems is dependent on what information was put in them, adding that unless authorities had a strong piece of information against somebody, they were not going to put restrictions on people in a free society.

Republican Senator Lindsey Graham said law enforcement should have kept a closer eye on Tsarnaev after the FBI spoke to him two years ago. The FBI should also have realized last week following the bombings that he was in databases, Graham told reporters.

"After the bomb went off, don't you think one of the first things the FBI would do is say, 'Have we interviewed anybody in the Boston area that may fit the profile of doing this?' How could his name not pop up, the older brother? And when you have the photo the whole world is looking at, how could we not match that photo with him already being in the system?" Graham said.

The Russians had something on this guy. They asked the FBI to investigate him even before he made his trip to Dagestan. We still don't know what the Russians told the FBI about Tamerlan Tsarnaev that might have triggered an investigation - even if it is now characterized as a "courtesy" to a friendly power.

All this makes you wonder if the FBI didn't have something a little more incriminating on Tamerlan Tsarnaev than what we've been told. And if that's true, Senator Graham's questions become even more urgent. Tamerlan Tsarnaev inadvertently exploited a hole in our security. Time to plug that hole before someone else slips through who might be able to do a lot more damage than these amateurs did.