The assessment also said "no credible, specific information indicating an imminent threat" to the race had been noted, Tribune Newspapers reported Friday.
"The FBI has not identified any specific lone offender or extremist group who pose a threat to the Boston Marathon," the report said.
The report was written by the Boston Regional Intelligence Center, a command center of federal and local law enforcement authorities and funded in part by the U.S. Department of Homeland Security.
In the aftermath of the April 15 bombings that killed three people and injured hundreds, what was known to whom when has become contentious. Federal officials said the warning showed they did their job, but Boston and Massachusetts law enforcement officials said during a congressional hearing Thursday the FBI didn't tell them about an investigation into Tamerlan Tsarnaev, one of two brothers suspected of carrying out the attack.
"We would have liked to have known," Edward Davis, Boston Police Department commissioner, told the House Homeland Security Committee in the first of several hearings investigating the attacks.
Tamerlan Tsarnaev, 26, was killed in a shootout with the police April 19, four days after the Boston Marathon bombings. His brother, 19-year-old Dzhokhar Tsarnaev, also suspected in the bombings, was captured later that day and is in a federal prison medical facility in Massachusetts.
Davis said he didn't know how local officials would have interpreted Tamerlan Tsarnaev's 2012 trip to Dagestan, noting that the FBI interviewed the 26-year-old but found nothing suspicious and that Russian officials did not tell the FBI why they were interested.
"We would certainly have looked at the information," Davis said. "We would certainly have talked to the individual."
But, he added, "I can't say I would have come to a different conclusion" than the FBI.
The Homeland Security threat assessment was dated April 10 and included a map of the Boylston Street finish line area, the area targeted by the bombers.
Davis said his officers would "absolutely" have taken a second look at Tsarnaev if they had known about the warning.
"My fear is that the Boston bombers succeeded because our system failed," said U.S. Rep. Mike McCaul, R-Texas.
The FBI said Boston police could have read the information about Tsarnaev as easily as any other Joint Terrorism Task Force member, noting the force "specifically had representatives assigned to the JTTF squad that conducted the 2011 assessment of deceased terrorism suspect Tamerlan Tsarnaev."
The FBI statement noted the Boston task force had conducted some 1,000 assessments in 2011, a workload that made it unlikely each assessment could get close attention from every task force member.
It also noted Tamerlan Tsarnaev lived in Cambridge, not Boston.