The House task force on the attempted assassinations of former President Donald J. Trump devolved into division on Thursday, after Republicans invited two members of Congress who have promoted conspiracy theories about the shooting to testify as witnesses during a hearing.

Democrats on the task force refused to attend an afternoon session of the panel during which Representatives Eli Crane of Arizona and Cory Mills of Florida, both Republicans and former military snipers, testified.

The two men have been conducting their own unauthorized investigation after they were not picked for seats on the official task force investigating the July 13 shooting at a Trump rally in Butler, Pa. They have promoted conspiracy theories that the shooting was an inside job or that it was carried out by more than one person instead of a lone gunman, Thomas Crooks, 20.

On Thursday, they encouraged the task force to explore the theory that there was a second gunman working with Mr. Crooks.

“I challenge you to continue to go forward and to not be feared away from the idea of being labeled a conspiracist by looking into every possible outcome,” Mr. Mills told the Republicans who remained in the room.

Representative Laurel Lee, Republican of Florida and a task force member, assured the men that the panel would investigate “all of the relevant theories.”