Authorities are investigating whether suspected shooter Thomas Matthew Crooks used a drone to surveil the rally site sometime before the shooting, according to two sources familiar with the situation.
ABC News' Josh Margolin and Luke Barr

House Homeland Security Committee Chairman Mark Green, R-TX, has subpoenaed Department of Homeland Security Secretary Alejandro Mayorkas for documents related to the assassination attempt on Trump, the committee said Friday.

The subpoena is for documents and information regarding the "failed security efforts," which Green had asked to receive by Friday, the committee said. The request includes the security plan for the site and communications regarding coordination efforts between DHS and local authorities.

The subpoena ordered Mayorkas to produce the materials to the committee by July 26.

A comprehensive timeline of the attempted assassination suggests the ingredients were there to have potentially thwarted the attack as late as six minutes before the first shot was fired.

The timeline, reviewed by ABC News, said a member of a local SWAT team from Beaver County, Pennsylvania, first spotted the suspect, Thomas Matthew Crooks, at 5:10 p.m. Saturday -- about one hour before the shooting -- when he walked by a window. The team was comprised of eight snipers and spotters and had been in position since 10:30 a.m.

At 5:14 p.m., a member of the Beaver County sniper team took Crooks’ photo.

At 5:28 p.m., he took a photo of a bicycle and backpack.

At 5:32 p.m., a sniper saw Crooks looking at news feeds on his phone and noticed he had a range finder. The sniper flagged the suspicious activity in a group text and received a response saying, "Call into command." The sniper did as instructed at 5:41 p.m. and communicated a description of the suspect lurking around the AGR Building, the roof of which Crooks ultimately used as a perch.

At about 6 p.m., police sought information about the suspect’s direction of travel. At 6:05 p.m., Crooks was spotted at picnic tables carrying a backpack, and that information was relayed over the radio.

At 6:05 p.m., Trump walked onto the stage.

At 6:06 p.m., the sniper who first noticed Crooks left his position and went to meet patrol officers to let them know the suspect was around the building on the side of the fairgrounds. One marked vehicle and an unmarked vehicle moved into position.

At 6:12 p.m., shots were fired.

Police climbed onto the roof and confirmed the suspect dead at 6:32 p.m. A search of Crooks’ pockets turned up a phone and a transmitter device.

The timeline does not reveal who received the information and whether concerns a suspicious person and possible threat were transmitted to Secret Service commanders on site, who could have delayed Trump’s entry into the rally or opted to take additional security steps.

-ABC News’ Sasha Pezenik, Josh Margolin, Aaron Katersky and Chris Looft

The body of Corey Comperatore, the firefighter who was killed at Saturday’s rally while shielding his family from the gunfire, was transported on a fire truck for his funeral in Pennsylvania on Friday.

His fellow firefighters came together to carry his casket.

Comperatore, 50, leaves behind a wife and two daughters.

"Corey Comperatore was our beloved father and husband, and a friend to so many throughout the Butler region," the family said in a statement on Thursday. "He was a local leader and veteran, a former fire chief, and a committed Christian who found peace and joy through our church.”

"We thank the countless people who have prayed for us throughout the past week," the family added.