Author Michelle R. McPhee's Maximum Harm. Excerpt from page 22:

Something was clearly going on in Cambridge that night that the feds did not want to share with local law enforcement officials. All week there had been whispers about arguments at the Black Falcon Cruise Terminal evidence center. There ten separate viewing stations with computer terminals had been set up along three rows to review the 655 videos that the FBI would later say had been collected as part of the investigation. Cops and agents sat side by side looking for anyone in the footage who looked out of place or nervous, or who was carrying a black backpack - the pieces of which had been the flesh of some of the marathon bombing victims and collected by FBI forensic examiners at hospitals all over the city. Off to one side, remembered one BPD homicide investigator, two FBI agents sat alone. They didn't introduce themselves. They didn't mingle. Instead they compared photos in their lap with photos on their computer screen, a detail cooborated by other witnesses who requested anonymity. The FBI had sent an expert from its Forensics Audio, Video, and Image Analysis Unit, Special Agent Anthony Imel, from his lab at Quantico, Virginia, to Boston to oversee the data collection. Imel did not appear to have any oversight of the FBI agents sitting by themselves, the witness said. A local FBI agent, Kevin Swindon, who supervised the Boston division's Computer Analysis Response Team, didn't either. He was too busy analyzing a security tape taken from inside the Forum restaurant, which in a clear and horrifying way showed the second blast. "We had numerous amounts of employees watching this video over and over and over again," he would later tell ABC News. "We couldn't see anything that stuck out."

One man from the Drug Enforcement Agency (DEA) finally could not take it any longer. He stood up and confronted the duo: "What are you guys looking at?" There was no response. A retired investigator who was there recalled in an interview conducted on background that the DEA agent said "Fuck you guys. You know who these mutts are and you're not sharing!" The agent stormed out. But his words stuck with the other officers and agents still looking at the videos. Those FBI agents were not seen at the evidence center again.

Multiple police officers assigned to work at the Black Falcon Cruise Terminal but not authorized to speak on the record told me: "They knew. They held it [the information] for days. They knew."