The Department of Homeland Security’s inspector general has opened an investigation into the Secret Service’s handling of the rally.

A new bulletin listed under "ongoing projects" on the DHS Office of Inspector General website said the project objective is to "evaluate the United States Secret Service's (Secret Service) process for securing former President Trump's July 13, 2024 campaign event."

Senators will receive an unclassified, all-senator briefing via phone call Wednesday afternoon, according to Senate Majority Leader Chuck Schumer’s office.

The Secret Service, FBI and Department of Justice will address the senators.

The House will also have an unclassified, all-member virtual briefing Wednesday afternoon, multiple sources said.

Authorities suspect Thomas Matthew Crooks, the man they allege tried to assassinate former President Trump, was able to access the roof by climbing over an air conditioning unit adjacent to the building, sources tell ABC News. Though law enforcement sources say Crooks is believed to have purchased a 5-foot ladder at a Home Depot before the shooting, it does not appear to have been used to climb on the roof and was not found at the scene.

Investigators found a remote transmitter in the suspect's pocket that may have been intended to set off two suspicious devices found in the suspect's car and one in his home, according to law enforcement sources.

Devices at both locations were similarly constructed in ammunition containers using components including receivers that appear to have been paired with the remote control found in the suspect's pocket, sources said.

The receivers in each device were connected to a series of components that investigators say met the "eye test" -- having parts present that could have made viable devices, although the functionality of the devices is still being determined. The purpose of the devices is also unknown. Whether they were intended to cause a significant blast effect and hurt people or if they were designed to create smoke, fire and a low-grade blast for a diversion is unclear.

Investigators also found a tactical vest in the suspect's car, though it is unclear why he did not wear it during the assault on the former president's rally. Some investigators are wondering whether he anticipated dying in the attack.

The shooter also asked his employer if he could take a day off on Saturday but said he would return to work perhaps as soon as Sunday, the day after this attack, according to law enforcement sources. But thus far, investigators have turned up nothing to suggest that he had any accomplices or other support to carry out his deadly objective.

An examination of his phone, which the FBI has now unlocked, has not turned up any significant information about the suspect's motives -- only what sources described as "routine" information for a 20-year-old male. Thus far, investigators have turned up nothing to suggest that he had any accomplices or other support to carry out his deadly objective.

-ABC News’ Jack Date, Luke Barr, Pierre Thomas, Aaron Katersky

Butler County Sheriff Michael Slupe spoke with ABC News on Tuesday about the response to the attempted assassination of former President Donald Trump.

When asked whether he heard radio traffic about a suspicious person at or near the rally ahead of the shooting, Slupe said, "I did not, and I wish I did."

Slupe said he attended Saturday’s rally along with a number of his deputies.

He said that his office does not have a team of snipers, but that there is an emergency service unit "attached to" the Butler County District Attorney's Office that is made up of officers from different agencies around the county.

Slupe said he did not know exactly how many of these emergency service officers were on the scene on Saturday.

Slupe defended the response by his deputies and called for the public to avoid passing judgment until the investigations conclude, noting that his office is not facilitating these probes.

"There's several investigations going on, and at the end of the day, we're all going to learn something from this," Slupe said.

-ABC News' Jared Kofsky and Tonya Simpson