A counter sniper flagged a suspicious man using a rangefinder to the US Secret Service some 20 minutes before a gunman opened fire at a Donald Trump rally, according to members of Congress briefed by law enforcement.

A clearer timeline of the events leading up to the assassination attempt has begun to emerge after closed briefings to lawmakers on Wednesday.

Local police had initially spotted the gunman, who was acting strangely, at Saturday’s Pennsylvania rally about an hour before the shooting, according to the briefings.

They lost him in the crowd before he was spotted again by the counter sniper.

The new information has raised more questions about why Trump’s would-be assassin was not stopped sooner and why the former president was allowed to appear on the rally stage.

Thomas Matthew Crooks, the 20-year-old suspected gunman, was reportedly spotted early by local police, who flagged him as a skinny young man who was behaving in a suspicious manner.

They notified other police agencies, including the Secret Service, via radio. At the time, Crooks did not appear to have a weapon. They then lost track of him.

“He was identified as a character of suspicion because [he had] a rangefinder as well as a backpack. And this was over an hour before the shooting actually occurred,” Wyoming Senator John Barrasso, who was present at the briefings, told Fox News.

“So, you would think over the course of that hour, you shouldn’t lose sight of the individual.”

Later, around 17:45 local time, Crooks was spotted again, this time by a counter sniper officer around the Agr International building – the one the gunman later scaled up to aim at Trump.

The officer reportedly took a picture of the gunman looking through the rangefinder, according to CBS News, the BBC’s news partner, and immediately radioed to a command post to report the sighting.

By 17:52 – 19 minutes before the shooting – the Secret Service was made aware that Crooks was spotted with a rangefinder, and disseminated that information to other officers on site, CBS reported.

A rangefinder is an instrument that can be used to help measure the distance to a target.

It was also revealed during the briefings that the gunman had visited the site of the attack, the Butler County fairgrounds, at least once in the days before the assassination attempt and had previously searched on his phone for symptoms of a depressive disorder, an official familiar with the briefing told CBS.

The attacker had also used his phone to search for images of both Donald Trump and President Joe Biden. FBI Director Wray told lawmakers on the call that more than 200 interviews had already been conducted and 14,000 images reviewed.

Crooks fired at Trump shortly after he began speaking at the rally at 18:11. One rallygoer was killed in the shooting and two others were injured. Trump was wounded in the ear.

Crooks was killed by Secret Service snipers within 26 seconds of opening fire.

In the briefing with law enforcement agencies, multiple Republican senators criticised the lack of transparency from investigators and expressed outrage that Trump was allowed to take the stage even after a threat was identified.

“I am appalled to learn that the Secret Service knew about a threat prior to President Trump walking on stage,” tweeted Senator Marsha Blackburn of Tennessee.

Some senators who participated in the call demanded the resignation of Secret Service Director Kimberly Cheatle.

“The egregious security failures and lack of transparency around the assassination attempt on President Trump demand an immediate change of leadership at the Secret Service,” tweeted Utah Senator Mike Lee.

Senate Minority Leader Mitch McConnell said after the call that “the nation deserves answers and accountability” and a change in leadership at Secret Service would be “an important step in that direction”.

House lawmakers similarly were briefed on Wednesday by law enforcement about security and what led up to the Saturday shooting.

Republican Speaker of the House Mike Johnson also called for Ms Cheatle to quit. He said he plans to open an investigation in the House.

“It’ll be comprised of Republicans and Democrats to get down to the bottom of this quickly, so the American people can get the answers that they deserve,” he told Fox News.

FBI Director Chris Wray, who participated in the calls, told lawmakers that no motive has yet been identified for the gunman.

Ms Cheatle, a 27-year veteran of the Secret Service, is due to testify next week to the Republican-controlled House Oversight Committee and House Homeland Security Committee.

She also gave a private, in-person briefing on Tuesday to Trump in Milwaukee regarding the assassination attempt, CBS reported, citing a person familiar with the meeting.

Ms Cheatle had previously said that the agency relied on local police to secure the building where the gunman climbed to the roof with his firearm.

Local law enforcement, however, had told Secret Service that they did not have enough resources to secure that building, according to Richard Goldinger, the district attorney in Butler County, where the rally took place.

A local officer did come face-to-face with the gunman on the roof moments before the attack, Butler Township Manager Tom Knights told CBS.

The officer was searching after reports about a suspicious person. He was hoisted on to the roof by another officer and saw the suspect pointing a weapon directly at him, Mr Knights said.

The officer was in a “defenceless” position and let go, falling to the ground. He then alerted others to the gunman. Moments later, the shooting started.

Jason Russell, a former Secret Service agent who has worked at campaigned rallies, said the disaster at last week’s rally likely resulted from a miscommunication about different officials’ responsibilities.

“It seems to me like there was just a miscommunication, but the reality is, if it’s a Secret Service event, anything that happens is our fault,” he told the BBC.

Mr Russell added he would have at least placed an obstacle between the building from where the gunman fired and where Trump was delivering his speech to block the line of sight.

The attack is being investigated by the Department of Homeland Security’s inspector general, and President Biden said he would direct an independent review be opened.