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.

Surveillance video captured the suspect placing the pipe bombs near the committees’ offices between 7:30 p.m. and 8:30 p.m. on Jan. 5, 2021. Nobody was hurt before the bombs were rendered safe, but the FBI says both devices could have been lethal.

Kamala Harris, then the Vice President-elect, was inside the DNC offices when the pipe bomb was found outside the building about 1:05 p.m. on Jan. 6. Before the bomb was deactivated, then-House Speaker Nancy Pelosi’s motorcade passed by the DNC building as she was evacuated from the Capitol, according to House Republicans.

This week, the FBI is releasing a minute-long video that shows the suspect sitting on a park bench outside the DNC before placing the first bomb there at about 7:54 p.m. The suspect placed the second bomb about 8:16 p.m., in an alley behind the RNC, the FBI says.

The FBI also is releasing closeup images of the type of Nike sneakers worn by the suspect. Fewer than 25,000 pairs of the same shoe were sold between August 2018 and January 2021, according to the FBI.

“Based on attire, those are probably the most remarkable or distinctive feature when it comes to clothing the suspect wore,” Sundberg said. “We’re hoping that somebody might recognize that.”

The FBI used surveillance footage to track the suspect’s movements through Capitol Hill on the night of Jan. 5. The suspect initially is captured on video at about 7:34 p.m. at the intersection of First Street and North Carolina Avenue. The suspect is last seen on camera around 8:18 p.m. heading east on Rumsey Court.

“The suspect in this case did a very good job covering themselves up at a time that this would not be abnormal, so it didn’t raise any attention,” Sundberg said, referring to face masking during the COVID-19 pandemic.

Video of the suspect was sporadically available and often of poor quality, according to Sundberg. The 17-hour gap between the planting and discovery of the pipe bombs made it more difficult to identify potential witnesses, he said.

Authorities previously offered a reward of up to $500,000 for information leading to the suspect’s arrest and conviction.

President-elect Trump, who returns to the White House on Jan. 20, repeatedly has vowed to pardon some or many of the rioters who stormed the Capitol. More than 1,500 people have been charged with Jan. 6-related crimes. About 1,100 have been convicted and sentenced. Over 700 defendants got terms of imprisonment ranging from a few days to 22 years.

Sundberg said he doesn’t expect the change in administration to alter the course of the FBI’s pipe bomb investigation.

“We don’t know who the suspect is,” he added. “And I expect that we will continue to work this case until its logical conclusion and we identify a suspect.”