The investigation began after a congressman in the US House of Representatives set off a fire alarm as he tried to delay a crucial budget vote on Saturday.
New York Democrat Jamal Bowman says it was an accident.
But his opponents accused him of trying to derail the vote, which was designed to avoid a shutdown of the US federal government.
The alarm triggered an hour-long evacuation. A deal was eventually agreed to avoid a shutdown.
The alarm sounded as Democrats sought to delay the vote as they sought more time to read the bill and decide whether to support it.
“Today, in a hurry to vote, I came to the door that was open to vote, but it won’t open today. I’m ashamed to admit that I set off the fire alarm, mistaking the door for opening.” Mr Bowman said in a statement.
“There is no attempt, in any way, to delay any vote,” he added.
“It’s the opposite — I tried to rush to vote, and I finally did, joining my colleagues in a bipartisan effort to open our government.”
But Republicans have accused him of deliberately trying to sabotage the vote.
Representative Brian Steele, a Republican from Wisconsin and chairman of the House Administration Committee, announced that the investigation is ongoing.
Republican Speaker Kevin McCarthy called it a “new low.”
“I am appalled at the actions of the Democrats today to delay, to shut down,” he said.
“We’ve seen how people are treated in this Capitol when they’ve done something wrong. It will be interesting to see how he’s treated and what he’s tried to prevent when it comes to the American public.”
The bill funds the government until November 17. President Joe Biden signed the legislation minutes before the midnight deadline.
The proposal comes as Mr McCarthy battles a rebellion by hardliners in his own party.
If Congress had failed to keep the government open, the shutdowns would have delayed pay for millions of federal employees and military personnel.