The father of the gunman who tried to assassinate Donald Trump called police before the Saturday shooting because he was concerned about his son, according to media reports.

The call is one of a number of red flags revealed in recent days that law enforcement was notified about before gunshots rang out at Trump’s Pennsylvania rally on Saturday. Law enforcement – specifically the US Secret Service – has faced mounting questions about security with calls by some lawmakers that the head of the agency should resign.

Matthew Crooks’ father called police because he was worried about his son and his whereabouts, a law enforcement source told the BBC’s news partner CBS. It’s unclear when the call was made but it was before the shooting.

It is unclear what his father told police. Fox News reported that Crooks’ parents, Mary and Matthew, told officers “they were worried” about their son and that he had disappeared without any advance notice.

His parents are both co-operating in the investigation, the FBI has said.

Law enforcement sources have told US media that the gunman had conducted online searches into a major depressive disorder and the Democratic National Convention scheduled for August.

He had also saved images of Trump, President Joe Biden, Attorney General Merrick Garland, FBI Director Chris Wray and a member of the British Royal Family, according to reports from the Washington Post and Associated Press.

Investigators are still trying to trace a potential motive for the 20-year-old gunman, who was shot dead by Secret Service snipers after opening fire. His attack left one member of the audience dead and several others wounded.

The preliminary investigation has found that Crooks climbed onto the roof of a nearby building outside the rally by climbing onto an air conditioning unit. The units were located next to the building, the unnamed official told CBS.

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A counter sniper flagged a suspicious man using a rangefinder to the US Secret Service some 20 minutes before the attack started, according to members of Congress briefed by law enforcement this week.

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

Local police initially spotted the gunman, who was acting strangely and had a backpack, about an hour before the shooting. They lost him in the crowd, but he was spotted again by the sniper.

Officers were alerted by radio about a suspicious person and searched the area where Crooks had perched his rifle on a rooftop.

Finding no one, one officer decided to check the roof. The officer was hoisted on to the roof by a colleague and came face-to-face with the suspect, Butler Township Manager Tom Knights told CBS.

The suspect pointed a rifle at him and the officer, who was in a “defenceless” position, let go from the roof and fell to the ground.

He then alerted others to the gunman. Moments later, the shooting started.

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No weapon was spotted by law enforcement when Crooks was seen in the crowd and officials are trying to determine how no one saw his AR-style rifle.

Investigators are examining various theories, including that he had stashed it earlier in the day near the air conditioning units or that he was somehow able to smuggle it inside his backpack.

Retracing his footsteps in the hours before the attack will be key to understanding how the shooting unfolded, officials say.

Officials told CBS that the semi-automatic rifle he used had been legally purchased by Crook’s father in 2013.

When the gunman was found, he was carrying a remote detonator and his car contained explosives, law enforcement sources have told US media.

It continues to remain unclear what motivated the attack, and whether any political ideology is to blame.

• Around 17:11: local officers spot Crooks and notify other law enforcement but then lose track of him, according to briefings between police and lawmakers
• 17:45: A counter sniper officer calls in with a report and a photo of a man – who turned out to be Crooks – acting suspiciously around a building near the rally, according to local media reports
• 17:52: US Secret Service become aware of a suspicious person with a rangefinder on the ground, according to sources familiar with the briefing to lawmakers
• 18:03: Trump begins speaking at the rally
• Around 18:09: Rallygoers spot Crooks on the roof and attempt to tell law enforcement
• 18:11: Crooks opens fire. He is fatally shot by Secret Service counter snipers 26 seconds later