The FBI has made a significant breakthrough in a nearly five-year investigation by arresting a suspect linked to the pipe bombs planted in Washington, D.C., on the eve of the January 6, 2021, Capitol riots, according to a law enforcement official who spoke with The Associated Press.

This arrest is the first major development in a case that has puzzled investigators and fueled numerous conspiracy theories. It sheds light on an enduring mystery from the dark chapter of American history marked by the violent breach of the U.S. Capitol.

The official, who requested anonymity due to the sensitive nature of the ongoing investigation, revealed that the arrest occurred on Thursday morning. The suspect is reportedly a male, though no further details, including potential charges, have been disclosed at this time.

The pipe bombs were discovered on January 5, 2021, strategically placed near the headquarters of the Democratic and Republican National Committees in Washington, D.C. While the bombs did not cause any injuries and were safely defused, the FBI has noted that both devices had the potential to be deadly.

Since the incident, authorities have sought public assistance to identify the individual captured on surveillance footage. Investigators have grappled with basic questions about the suspect’s identity, including their gender, motives, and whether there was a direct link to the Capitol riot that followed. On January 6, supporters of then-President Donald Trump stormed the Capitol in an attempt to overturn the certification of his 2020 election loss to Joe Biden.

Seeking a breakthrough, the FBI last January publicised additional information about the investigation, including an estimate that the suspect was about 1.7 metres, 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 politicians and right-wing media outlets promoted conspiracy theories about the pipe bombs.

House Republicans also criticised 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 utilise crowd-sourcing leads,” he wrote in a long post on X last month.