As investigators begin to unravel the Boston Marathon bomb plot, a separate inquiry is unfolding over the FBI's handling of suspected attacker and possible mastermind Tamerlan Tsarnaev.

One thing is certain: The FBI at least knew Tamerlan's name before the attacks last week that killed three and left more than 260 injured. (A fourth casualty, a police officer, was allegedly shot dead by Tamerlan and/or his brother Dzhokhar shortly before their capture.) But beyond that, it's less clear exactly what the agency knew and when, lending fire to critics who want to know if the agency missed red flags that should have alerted the FBI to the suspects before the attack.

Soon after authorities identified the two suspects, it was revealed that a foreign government — now known to be Russia — had asked the FBI in 2011 for information about any potential links between Tamerlan and terrorist groups. The FBI said it had checked up on Tamerlan and found nothing alarming, so the agency moved on. 

How the FBI could have moved on so quickly prompted calls from some lawmakers for more details about that initial investigation. The FBI's response: The agency asked its Russian counterparts for more information to conduct a deeper search, but the Russians never responded. Of its own accord, the FBI then sent agents to interview Tamerlan and his family, determined he posed no threat, and wiped its hands clean of the matter. The FBI also says it had no legal authority to do anything else at that time.

Cut and dry, right?

Except Russia may have actually warned U.S. authorities about Tamerlan not once, but multiple times — directly contradicting the FBI's claim that a lack of Russian input led to the case being dropped. Sen. Richard Burr (R-N.C.), who along with other members of the Senate Intelligence Committee was briefed Tuesday on the FBI's investigation, said Russia had repeatedly warned the FBI about Tamerlan, including at least one time since October 2011. The FBI has said it interviewed Tamerlan in the summer of 2011.

Relatedly, there are also questions about how Tamerlan went unnoticed despite being included on multiple government threat lists. After FBI agents interviewed him, his name was added to the agency's Terrorist Screening Database (TDSB), according to CNN. His name also went on at least two other federal anti-terror lists, including a classified federal database of "potential terrorists," called TIDE, short for Terrorist Identities Datamart Environment. 

But that database houses over half a million names, making it unwieldy. According to Reuters, Tamerlan's name only wound up there because the FBI had previously spoken to him, and inclusion on the list doesn't automatically mean the agency is actively monitoring someone. Plus, TIDE is only one of many interrelated anti-terror databases.

From Reuters' Mark Hosenball:

The TIDE database is one of many federal security databases set up after the September 11, 2001, attacks. The database system has been criticized in the past for being too cumbersome, especially in light of an attempted attack on a plane in 2009. Intelligence and security agencies acknowledged in Congress that they had missed clues to the Detroit 'underpants bomber' Umar Farouk Abdulmutallab. Officials said after the incident that he had been listed in the TIDE database. [Reuters]

While Tamerlan wound up in a few databases, he was not placed on more serious watch lists such as the "no-fly" list that would have prevented him from traveling to Russia last year. So when he made that trip, a Homeland Security database merely flagged his name.

However, by the time he returned six months later, he'd slipped off the government's radar.

"Yes, the system pinged when he was leaving the United States. By the time he returned, all investigations — the matter had been closed," Homeland Security head Janet Napolitano told senators Tuesday.

It's unclear exactly what happened there. Sen. Lindsey Graham (R-S.C.) blamed a misspelling of Tamerlan's name. Napolitano has shot that theory down, saying the department knew of his trip to Russia despite the misspelling. A more likely explanation is that, since the FBI never considered him a real threat in the first place, the government had to eventually close the book on him. 

Here's the National Journal's Major Garrett explaining that point:

Tamerlan’s questioning was carried out under very precise procedures found in the Justice Department's Domestic Investigations Operations Guide, or DIOG…The guide allowed FBI agents to undertake an assessment of an individual, but it also spelled out how far that assessment could go and how long it could last if the trail of evidence ran cold or never existed in the first place. Agents had 90 days to keep a file open or close it. If there is no 'derogatory information,' the file must be closed, and its very existence is not enough to subsequently prejudice the government against the individual. [National Journal]

In short, since the feds had no reason to believe Tamerlan was dangerous, they eventually had to stop watching him. That cutoff point, apparently, coincided with his trip to Russia.

Given all the confusion, lawmakers are now looking more broadly at whether the FBI and Homeland Security failed to "connect the dots" because of poor information-sharing between agencies and the convoluted system of separate watch lists — a conversation that has cropped up after every terror plot since 9/11, with seemingly no resolution. Sen. Susan Collins (R-Maine) voiced that concern after being briefed on the investigation, citing "serious problems" with inter-agency information.

"That is troubling to me that this many years after the attacks on our country in 2001, that we still seem to have stovepipes that prevent information from being shared effectively, not only among agencies but also within the same agency, in one case," she told reporters. 

UPDATE: 8:45 p.m. ET: 
According to the New York Times, both the CIA and FBI placed Tamerlan Tsarnaev on government watch lists prior to the Boston attack. The CIA, following a request from Russia for information on Tamerlan, had him placed on TIDE. The FBI, meanwhile, added his name to the Treasury Enforcement Communications System, which automatically notified customs when he left the country. Previously, it was known that Tamerlan had been placed on multiple watch lists, though it was unclear who had added his name to those databases.