Members of the newly formed House task force on 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.”

“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.”

Speaker Mike Johnson left Mr. Crane and Mr. Mills, Republican of Florida — both of whom are trained as military snipers — off the panel, a decision that Mr. Crane called “political” and that he said indicated that the congressional task force might try to engage in a cover-up.

It suggested, he said on the podcast, that “not only are the witnesses that are called going to be politically selected, but what information is released to the public is also going to be political as well.” He said his group would be holding a hearing soon.

Mr. Crane has made the baseless suggestion that the shooting was part of a coordinated campaign by Democrats or shadowy government actors to stop Mr. Trump from being re-elected. Mr. Mills has said that he had a hard time believing the security breaches that preceded the assassination attempt were not “intentional as opposed to fecklessness.”

While no evidence has surfaced that anyone other than a lone gunman was involved, one member of the task force, Representative Mike Waltz, Republican of Florida, has questioned whether Mr. Crooks acted alone. Mr. Waltz did not attend the site visit on Monday, but other members of the panel said they had uncovered no indication there was a co-conspirator.

“I don’t think anyone on the task force has seen any hard evidence that would suggest that would be the case,” said Representative Jared Moskowitz, Democrat of Florida.

The lawmakers on the task force said they were interviewing local law enforcement officials during their site visit. Representative Mike Kelly, Republican of Pennsylvania and the chairman, and Mr. Crow have sent out joint letters to the Department of Homeland Security, Secret Service, Justice Department and F.B.I., demanding documents and interviews.

But Mr. Kelly said there was no substitute for being physically present at the site in Butler, which is part of his congressional district.

“It’s the difference between day and night,” he said. “When you’re actually here on this surface, when you’re actually walking these grounds, when you’re actually going to the building, when you’re actually up on the roof.”

Representative Laurel Lee, Republican of Florida, said the group had received a briefing from the F.B.I. and the Secret Service and was beginning to work through a litany of interviews, including state and local law enforcement officials involved in security on the day of the shooting.

Mr. Moskowitz said he left the site visit with a greater sense of outrage at the Secret Service’s failures that day.

“Being here and seeing the proximity of the buildings, it reminds me, quite frankly, how outrageous it was that the former director of Secret Service did not come here to get a sense of what this looked like,” he said, adding: “We were inches away from an American catastrophe.”

Annie Karni contributed reporting.