Members of the newly formed House task force to investigate the attempted assassination of former President Donald J. Trump took one of their first investigative steps on Monday, traveling to the site of the shooting in Butler, Pa., to scrutinize the scene.

Lawmakers who walked the grounds of the Butler Farm Show, including climbing onto the roof where Thomas Crooks, 20, opened fire at a campaign rally, said they were shocked and appalled that the Secret Service had apparently left Mr. Trump unprotected against multiple potential lines of fire.

“You always secure the high ground, or you have eyes on the high ground,” said Representative Jason Crow of Colorado, the top Democrat on the task force and a former Army Ranger. “I definitely took note today that there were a lot of lines of sight that appear to have been unsecured that day, that didn’t have eyes on or that weren’t secured.”

But even as the Republicans and Democrats said they were pushing ahead on their bipartisan investigation, authorized last month by a unanimous vote of the House, a small but vocal band of right-wing lawmakers who have circulated conspiracy theories about the shooting said they were conducting their own simultaneous inquiry.

“So myself and Cory Mills are leading a parallel investigation into what happened on J-13th and the assassination attempt,” Representative Eli Crane, Republican of Arizona, said last week on the War Room Podcast, referring to the July 13 shooting. “And we’re doing that because, like many of your listeners, we don’t trust the federal government to actually do the job necessary.”