Police in charge of protecting Congress warned of a possible plot by a militia group to breach the U.S. Capitol on Thursday amid lingering security concerns following the attack on the building in January.

Noting that the intelligence on the plot is taken "seriously," the U.S. Capitol Police Department said Wednesday it has "already made significant security upgrades" including boosting manpower to ensure the protection of Congress.

Police officers stand guard in front of the Capitol in Washington on Jan. 20, 2021, for U.S. President Joe Biden's inauguration ceremony. (Kyodo) ==Kyodo

The police said in a press release that the militia group in question is "identified," but did not elaborate.

The Capitol was attacked on Jan. 6 by a mob supporting then President Donald Trump, which included right-wing extremists. They stormed the building as Congress was engaged in a process to formally certify President Joe Biden's election win.

According to U.S. media, a right-wing conspiracy theory is circulating that Trump will be sworn in Thursday for a second term in office. The date March 4 had been the original inauguration day for presidents before 1933.

Trump was impeached by the House of Representatives just days before the end of his term on Jan. 20 over a charge of inciting insurrection at the Capitol, such as through his baseless claims of election fraud.

He was acquitted in a subsequent trial at the Senate, as those calling for conviction did not reach the required two-thirds majority.

Five people died in the Capitol riot, including a police officer and a Trump supporter who was shot by police.