"We write today to request an immediate investigation into the suspicious behavior and access given to visitors to the Capitol Complex on Tuesday, January 5, 2021 - the day before the attacks on the Capitol," said the members of Congress in the letter.
The members who signed the letter, including former military service members who are "trained to recognize suspicious activity," stated that they say an "extremely high number of outside groups" in the Capitol on Tuesday, which they found unusual for several reasons, including coronavirus restrictions on public tours. The tours were reported to the Sergeant at Arms within the same day.
In a video address on Facebook, Sherill called the tours "reconnaissance for the next day."
"Let's be clear, there's no way those groups could have gotten into the Capitol without a member of Congress or a staff member of a member of Congress," Sherrill told USA Today on Thursday, adding that the Capitol complex is difficult to navigate and it seemed that the offices of certain members of Congress were targeted.
"There's just this sense from members of Congress that they had an inside understanding of the Capitol complex, which was chilling: that they had done some sort of reconnaissance, or had inside information about the layout," said Sherrill.
The letter added that the visitors seen by members of Congress "appeared to be associated with the rally at the White House the following day," without explaining further what that meant.
"Members of the group that attacked the Capitol seemed to have an unusually detailed knowledge of the layout of the Capitol Complex. The presence of these groups within the Capitol Complex was indeed suspicious," wrote the members of Congress, stressing that "it is important that we feel safe in the halls of Congress."
They requested that the security officials provide answers to a number of questions concerning security procedures at the Capitol and an investigation into the groups that visited the day before the riots.
REPUBLICAN REP. Vern Buchanan told USA Today that he found Sherrill's claims of conspiracy "startling and hard to believe."
"If she has evidence that a member of Congress conducted 'reconnaissance tours' to help the rioters plan last week's attack, she needs to release it immediately," said Buchanan. "I saw nothing of the kind when I was on the House floor that week and would be shocked to learn it was true."
US Capitol Police confirmed that they were investigating the allegations by Sherrill and the members of Congress who signed the letter.
On Friday, House of Representatives Speaker Nancy Pelosi stated that any members of the US Congress who helped a crowd of President Donald Trump's supporters storm the Capitol should face criminal prosecution.
Investigators are pursuing the possibility that the attack on the Capitol was planned ahead of time and was not just a protest that escalated, according to CNN. They are reportedly using counterterrorism strategies in some of the arrests connected to the insurrection.
Former FBI Director James Comey told CNN that there was "no doubt" that there were at least some participants who arrived ready for violence. "People wandering around exercising their First Amendment rights don't bring ropes and ladders and sledgehammers to a spontaneous event," said Comey. "This was a planned assault as if going after a castle."
Video reportedly filmed during the attack on the Capitol published by CNN showed a woman with a loudspeaker informing rioters of the layout of the building, going as far as detailing the number of doors and where they open to in some rooms.
"People should probably coordinate together if you're going to take this building," the woman can be heard saying in the video.
The FBI has warned armed protests are being planned for Washington and in 50 US state capital cities ahead of President-elect Joe Biden's January 20 inauguration, ABC News reported last week.
Reuters contributed to this report.