Davis said that none of four people he had assigned to the FBI's Joint Terrorism Task Force was aware that the FBI investigated the vague warning, found nothing and had closed the file. One of his detectives was in the dark despite being assigned to the unit that investigated Tsarnaev, Davis said.
"They tell me they received no word about that individual prior to the bombing," Davis said.
Davis said he would have liked to have known but conceded that it might not have prevented the attack. The commissioner said his detectives would have wanted to interview Tsarnaev.
"The FBI did that and they closed the case out," he said. "I can't say I would've come to a different conclusion based on the information at the time."
The House Homeland Security Committee hearing came less than three weeks after Tsarnaev died in a police shootout. His brother, Dzhokhar Tsarnaev, was arrested and faces federal terrorism charges.
The committee chairman, Rep. Michael McCaul, R-Texas, said the hearing will be the first in a series to review the government's initial response, ask what information authorities received about the brothers before the bombings and whether they handled it correctly.
Thursday's hearing was unlikely to shed much light on those questions. Nobody from the federal government testified.
But in a time of widespread budget cuts, the hearing began laying the groundwork for an expected push for more counterterrorism money. Both Davis and Kurt Schwartz, the Massachusetts homeland security chief, praised federal grants that for years have kept cities flush with money for equipment and manpower.
"People are alive today" because of money for training and equipment, Schwartz said.
McCaul and Rep. Bennie Thompson, the top Democrat on the committee, also spoke of the importance of federal money, as did former Sen. Joe Lieberman, one of the founders of the Department of Homeland Security, who took a new seat as a congressional witness.
"You can't fight this war without resources," Lieberman said.
Lieberman said it would have been possible, albeit difficult, to have prevented the bombing. He said the U.S. should have shared threat information with state and local law enforcement.
"When you're dealing with homegrown radicals, the community around them is going to be your first line of defense," Lieberman said. "State and local law enforcement will always have a better knowledge of the neighborhood, the institutions the people are going to be involved with."
In written testimony, Davis told lawmakers that cities should look at deploying more undercover officers and special police units and installing more surveillance cameras — but not at the expense of civil liberties.
"I do not endorse actions that move Boston and our nation into a police state mentality, with surveillance cameras attached to every light pole in the city," Davis said. "We do not and cannot live in a protective enclosure because of the actions of extremists who seek to disrupt our way of life."
Investigators used surveillance video from a restaurant near one of the explosions to help identify the Tsarnaev brothers.
"Images from cameras do not lie. They do not forget," Davis said. "They can be viewed by a jury as evidence of what occurred. These efforts are not intended to chill or stifle free speech, but rather to protect the integrity and freedom of that speech and to protect the rights of victims and suspects alike."
Copyright 2013 The Associated Press.