WASHINGTON — A harrowing chapter in American history remains shrouded in mystery: Who planted pipe bombs outside offices of the Democratic and Republican national committees in Washington on the eve of the attack on the Capitol?

Hoping to generate new tips from the public, the FBI is releasing more information about its pipe bomb investigation, including an estimate that the unidentified suspect is about 5 feet 7 inches tall. The bureau also is posting previously unreleased video of the suspect placing one of the bombs.

A host of basic questions remains unanswered four years later. For starters, investigators haven’t determined if the suspect is a man or a woman. Nor have they established a clear link between the pipe bombs and the Jan. 6, 2021, riot at the Capitol by a mob of Donald Trump’s supporters.

The FBI “can’t work on assumptions,” said David Sundberg, assistant director in charge of the bureau’s Washington field office.

“Without being able to confirm the suspect’s identity, it is very hard to definitively establish motive,” Sundberg told The Associated Press. “Therefore, it would be difficult for us to state that there is a link, although we can’t state there is not one.”

In the absence of harder evidence, Republican lawmakers and right-wing media outlets have promoted conspiracy theories about the pipe bombs. House Republicans also have criticized security lapses, questioning how law enforcement failed to detect the bombs for 17 hours.

“We remain focused on conducting an investigation using all of the tools we have at our disposal,” Sundberg said. “But it is incumbent upon us to follow facts and evidence.”

The FBI has assessed over 600 tips, reviewed about 39,000 video files and conducted more than 1,000 interviews over the past four years. Images show the suspect was wearing a gray hooded sweatshirt, a face mask, black gloves and a black and grey pair of Nike Air Max Speed Turf shoes with a yellow logo. The person also wore or carried a backpack containing the bombs.