WASHINGTON — Donald Trump's would-be assassin was flagged as "suspicious" by the Secret Service up to an hour before he began shooting but was lost in the crowd, lawmakers have been told by law enforcement officials.

In two closed briefings to lawmakers in the House and Senate on Wednesday, law enforcement officials, including the Secret Service, shared limited new information about security and the man who opened fire at a Trump rally in Pennsylvania on Saturday.

Wyoming Senator John Barrasso said the Secret Service told them they had spotted the attacker one hour before the attack, but then lost sight 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,” he told Fox News.

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

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 News, the BBC's news partner.

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.

However, multiple Republican senators criticized the lack of transparency from investigators on their call 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.

A law enforcement official involved in the investigation told CBS that a sniper from a local tactical team deployed to assist the Secret Service took a picture of the gunman looking through the rangefinder, and immediately radioed to a command post to report the sighting.

According to ABC News and other US outlets, the 20-year-old gunman was spotted again on the roof of a building 20 minutes before the attack began, officials revealed.

He was killed by Secret Service snipers within 26 seconds of opening fire on Trump.

Multiple senators who participated in the call complained that investigators did not answer their questions and 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.

Wisconsin Senator Ron Johnson called the briefing to lawmakers "unbelievably uninformative" and said investigators only took four questions from lawmakers.

Other senior Republicans also called for Ms Cheatle to resign. 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 has said the agency relied on local police to secure the building where the gunman climbed to the roof and perched his rifle.

A local officer came 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 rifle directly at him, 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.

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. — BBC