Law enforcement officials investigating the assassination attempt on Donald Trump told lawmakers today in an unclassified briefing that 20 minutes passed between the time Secret Service snipers first spotted Crooks on the roof and the time shots were fired at the former president, according to several law enforcement officials and lawmakers briefed on the matter.

According to this source, briefers said the snipers spotted him on the roof at 5:52 p.m. ET. The shooting happened at 6:12 p.m., 20 minutes later.

The briefing was conducted by the FBI Director, his Deputy Director and the head of the USSS.

In addition, briefers told lawmakers Crooks was identified as a person of interest a full 62 minutes before the shooting took place.

From the time he fired his first shot to Crooks being killed was just 26 seconds, according to law enforcement officials. Eleven seconds after the first shot, USSS counter snipers acquired their target -- and 15 seconds after that, Crooks was shot dead.

Former President Donald Trump is not expected to attend the funeral of Corey Comperatore, the 50-year-old spectator who was shot and killed at Saturday's rally, sources told ABC News.

Comperatore, a firefighter, died shielding his family from the gunfire, officials said. He’s survived by his wife and two daughters.

Comperatore’s obituary said friends can pay their respects on Thursday. Funeral services will be private, the obituary said.

-ABC News’ Soorin Kim, Kelsey Walsh and Lalee Ibssa

New video obtained exclusively by Pittsburgh ABC affiliate WTAE appears to show gunman Thomas Matthew Crooks pacing just beyond the boundaries of the Trump rally at 5:06 p.m. Saturday -- about 1 hour and 5 minutes before shots were first fired. Law enforcement sources said they believe this video shows Crooks.

House Homeland Security Committee Chairman Mark Green, R-Tenn., has invited several members of Pennsylvania state and local law enforcement to testify before the committee in the "coming days."

The invited law enforcement officers are: Pennsylvania State Police Commissioner Col. Christopher Paris, Butler County Sheriff Michael Slupe and Butler Township Police Lt. Matthew Pearson of the Butler Township Police Department.