The FBI made an arrest on Thursday in its nearly 5-year-old investigation into who placed pipe bombs in Washington on the eve of the Jan. 6, 2021, riot at the U.S. Capitol, a law enforcement official told The Associated Press.

The arrest marks the first time investigators have settled on a suspect in an act that had long vexed law enforcement, spawned a multitude of conspiracy theories and remained an enduring mystery in the shadow of the dark chapter of American history that is the violent Capitol siege.

The official who described the arrest was not authorized to publicly discuss a case that has not yet been made public and spoke on the condition of anonymity. The arrest took place Thursday morning, and the suspect is a man, the official said. No other details were immediately available, including the charges the man might face.

The pipe bombs were placed on the evening of Jan. 5, 2021, near the offices of the Democratic and Republican national committees in the District of Columbia. Nobody was hurt before the bombs were rendered safe, but the FBI has said both devices could have been lethal.

In the years since, investigators have sought the public’s help in identifying a shadowy subject seen on surveillance camera even as they struggled to determine answers to basic questions, including the person’s gender and motive and whether the act had a clear connection to the riot at the Capitol a day later when supporters of President Donald Trump stormed the building in a bid to halt the certification of the Republican's 2020 election loss to Democrat Joe Biden.

Seeking a breakthrough, the FBI last January publicized additional information about the investigation, including an estimate that the suspect was about 5-foot-7, as well as previously unreleased video of the suspect placing one of the bombs.

The bureau had for years struggled to pinpoint a suspect despite hundreds of tips, a review of tens of thousands of video files and a significant number of interviews.

In the absence of harder evidence, Republican lawmakers and right-wing media outlets promoted conspiracy theories about the pipe bombs. House Republicans also criticized security lapses, questioning how law enforcement failed to detect the bombs for 17 hours. Dan Bongino, the current FBI deputy director, floated the possibility last year before being tapped for his job that the act was an “inside job” and involved a “massive cover-up.”

But since arriving at the FBI in March, he has sought to deliver action to a restive base on the far right by promising that the pipe bombs investigation would be a top priority and defending the bureau’s work.

“We brought in new personnel to take a look at the case, we flew in police officers and detectives working as TFOs (task force officers) to review FBI work, we conducted multiple internal reviews, held countless in person and SVTC meetings with investigative team members, we dramatically increased investigative resources, and we increased the public award for information in the case to utilize crowd-sourcing leads,” he wrote in a long post on X last month.