The FBI made an arrest on Thursday in its nearly five-year-old investigation into who placed pipe bombs in Washington on the eve of the January 6 riot at the US Capitol in 2021.

The arrest marks the first time investigators have settled on a suspect in an act that had long vexed police, 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 arrest took place on Thursday morning and the suspect is a man, a police official said. The suspect was arrested in Virginia, according to another person familiar with the matter. No other details were immediately available, including the charges the man might face.

The people who described the arrest were not authorised to publicly discuss a case that has not yet been made public and spoke on the condition of anonymity.

The pipe bombs were placed on the evening of January 5, 2021, near the offices of the Democratic and Republican national committees in the District of Columbia.