Thomas Matthew Crooks, the gunman who tried to assassinate Donald Trump a week ago, was able to fly a camera-equipped drone over the fairgrounds near Butler, Pennsylvania, shortly before the former president was set to speak there, according to news reports.

The latest disclosure about security lapses that preceded the shooting comes as a more complete picture of Crooks’ preparations is emerging, though it still lacks any definitive motive for the 20-year-old’s actions that led to Trump being grazed by a bullet, the shooting death of former fire chief Corey Comperatore and the critical wounding of two rally-goers.

The Wall Street Journal, which cited law enforcement officials, said Crooks flew the drone on a programmed flight path earlier on the day of the shootings – 13 July – on a predetermined path over the event site.

Later in the day, the would-be assassin fired at least six rounds from a semi-automatic rifle from the roof of the American Glass Research building roughly 150 yards from where Trump was speaking. Soon after, Crooks was killed by a Secret Service counter-sniper with a single bullet to the head.

But investigators have said that Crooks was identified as a suspicious person more than an hour before the shooting when police officers saw him loitering outside the rally with a range finder and a backpack but had lost track of him.

Investigators now say they believe Crooks began planning the attack days after the Trump campaign announced the rally on 3 July and later scoped out the fairgrounds as many as six times in advance of the rally.

On the day of the rally, police saw “someone engaged in suspicious activity”, said representative Gary Palmer, a Republican from Alabama, who was briefed by law enforcement last week.

Oklahoma senator Markwayne Mullin, who was also briefed, said police “were actively looking for him for 19 minutes before the shots rang out”.

New information about Crooks’ intensive planning for the attack has also been gleaned from 14,000 browser history links in his phone. While he did not leave an ideological manifesto common to many mass-shooting perpetrators, FBI investigators have disclosed that online searches linked in his phone showed that he’d researched school shootings. He reportedly searched Michigan school shooter Ethan Crumbley and had a mugshot of him on his phone.

Crooks also performed internet searches on next month’s Democratic convention and Joe Biden, depressive disorder and explosive materials and chemical compounds. Crooks brought a pair of homemade bombs to the rally designed to be set off with a remote fireworks igniter, as well as a bulletproof vest and three 30-round magazines later found in his Hyundai Sonata.

Officials also disclosed that Crooks had received several packages to his home marked “hazardous materials”.

But little if any partisan ideological context or motive has been ascribed to the gunman. Mullin said Crooks “hated politicians as a whole”. Crooks’ former classmates at Bethel Park high school outside Pittsburgh recalled him being a quiet student with a small friend group, though accounts of his personality and school experience often vary.

Crooks excelled at math and had earned an associate’s degree in engineering science from the Community College of Allegheny County in May and had talked about becoming a mechanical engineer.

Since graduating, he’d worked at a Pittsburgh nursing home serving meals and washing dishes for $16 an hour, and liked to build computers, play video games and practice target shooting at a nearby gun-range, including on the day before the shooting. He told the nursing home he’d be back at work on Sunday.

Xavier Harmon, who taught Crooks computer technology, told the New York Times that he was “struggling” to make sense of his student. Like others in computer class, Harmon said, Crooks “didn’t feel like they were accepted among their peers, so computer technology was their place they called home”.

In an autobiographical statement Crooks wrote for his induction into the National Technical Honor Society in 2021, he said his interests “are highly varied, and include computer technology, engineering, history and economics”.

The need to ascribe political motive to Crooks’ assassination attempt may be misguided, experts in the field of mass shootings have said. “What we might be seeing here is, this was somebody intent on perpetrating mass violence, and they happened to pick a political rally,” James Densley, founder of the Violence Project, told the New York Times on Saturday.

An emerging picture depicts Crooks’ family as insular and anti-social. Both of his parents, Matthew and Mary, worked from home as licensed social workers and the FBI has said their small home was cluttered similarly to a hoarder’s house.

Neighbors said the family rarely initiated conversations. “He didn’t speak to anyone, and no one spoke to him,” recalled Liam Campbell, 17, who rode on the school bus with Crooks, to the Times. “He seemed like the kind of person who didn’t like to start conversations with people he didn’t know. He seemed nervous.”

Crooks’ guidance counsellor Jim Knapp said Crooks was more pre-occupied with the latest technology news or cryptocurrency than anything political. When asked about his weekend, Knapp told the Times, “Tom always had something like: ‘Well, I sat in my bedroom, and I was gaming. I was on my computer. I didn’t do much this weekend, but I still had fun.”

“Other than his drive for academics, Tom was simple,” he added.