Secret Service Director Kimberly Cheatle will testify on Monday before the House Oversight Committee, according to agency spokesman Anthony Gugliemi.

"The Secret Service is fully accountable for the safety of its protectees," Gugliemi said in a statement. "We are committed to better understanding what happened before, during, and after the assassination attempt of former President Trump to ensure it never happens again. That includes complete cooperation with Congress, the FBI, and other relevant investigations."

In the days since the attack, Cheatle and the Secret Service have faced heightening scrutiny for failing to prevent the incident from happening, and even calls from some for Cheatle to resign. Cheatle said she would not resign from her role.

Days after announcing that he would open a whistleblower hotline for reports related to Saturday’s assassination attempt, Sen. Josh Hawley, R-Mo., wrote to Homeland Security Secretary Alejandro Mayorkas that whistleblowers have approached his office to flag the "loose" security posture of the event.

"For example, detection canines were not used to monitor entry and detect threats in the usual manner,” Hawley wrote on Friday. “Individuals without proper designations were able to gain access to backstage areas. Department personnel did not appropriately police the security buffer around the podium and were also not stationed at regular intervals around the event’s security perimeter.”

Hawley also said whistleblowers told his office that the majority of staff at the rally were from the Homeland Security Investigations team, not the Secret Service.

-ABC News’ Allison Pecorin

Using an encrypted email address and cell phone number, someone using the name Thomas Matthew Crooks signed up online to attend Trump's Pennsylvania rally one week before the shooting, a source familiar with the matter told ABC News.

The FBI told lawmakers they've conducted 200 interviews as part of the investigation, a source said.

A search of Crooks' phone history revealed no indication of his political views, and investigators were still trying to determine his motive.

An FBI spokesperson did not respond to an email from ABC News seeking comment.

-ABC News' Will Steakin and Mike Levine

The suspected gunman who attempted to assassinate former President Donald Trump at his campaign rally over the weekend had visited and left the event site in Butler, Pennsylvania, before returning that evening, sources told ABC News.

Crooks came to the rally site on the morning of the shooting, stayed there for an hour, left and then came back sometime in the early 5 p.m. ET hour, sources said.