WASHINGTON (AP) — After a gunman killed 20 first-graders and six educators at Sandy Hook Elementary School seven years ago, the FBI released 1,500 pages of documents from its investigation.

Its report on the 2017 massacre in Las Vegas was just three pages.

The brevity prompted disbelief and outrage from survivors and relatives of victims. Even President Donald Trump said he was disappointed and surprised, though he said he understood the FBI had not determined a motive in the Vegas shooting.

“I don’t think they did that thorough of an investigation,” said Christine Caria, who survived the attack that killed 58 people and injured nearly 900 others. “It seems like it was really fast.”

Close to 16 months after the shooting, the FBI’s long-awaited report — released Tuesday — did little to shed light on the investigation and left its main question about motive unanswered.

“They were unable to find a real reason, other than that obviously he was sick, and they didn’t know it,” Trump said in an interview with The Daily Caller. “So, I was a little surprised and a lot disappointed that they weren’t able to find the reason, because you’d like to find a reason for that and stop it.”

The report compiled by the FBI’s Behavioral Analysis Unit found that gunman Stephen Paddock sought notoriety in the attack, but it cited no “single or clear motivating factor” to explain why he opened fire from his suite in a high-rise casino hotel on 22,000 people attending an open-air concert.

Aaron Rouse, special agent in charge of the FBI’s Las Vegas office, defended his agency’s handling of the investigation — calling it a “herculean” effort.

More than 1,000 FBI employees worked on the investigation, he said, adding that it would be a “mischaracterization” to deem the FBI’s inability to identify a specific motive as a failure.

“Everything that could be done to figure out why has been done,” Rouse said.

Sandra Breault, FBI spokeswoman in Las Vegas, did not immediately respond Thursday to messages about the president’s comments.

Jack Levin, author of several books on serial killings and professor emeritus of sociology and criminology at Northeastern University in Boston, said he doubted authorities had any reason to withhold information about Paddock.

But he said he would have liked the FBI report to detail whether a catastrophic loss might have triggered Paddock to undertake the attack.

“I’ve studied hundreds of cases, and in almost every case there has been a catastrophic loss,” Levin said. “Loss of a job, loss of lots of money, becoming deeply in debt, losing a relationship or a loved one, separation from important friends, an eviction.”

Mary Ellen O’Toole, who retired as an FBI profiler in 2009, said agents typically compile long reports after extensive analysis to help law enforcement and the public understand the investigation and what can be done to prevent future attacks.

“People really want more information in cases like this,” she said. “They want to know what we missed, who knew something and what was the motivation so this doesn’t happen anymore.”

The FBI said it enlisted experts who spent months examining several factors that might have prompted the rampage.

The report said the gunman was inspired in part by his father’s reputation as a bank robber who was once on the FBI’s most wanted list.

In many ways, Paddock was similar to other shooters the FBI has studied — motivated by a complex merging of developmental issues, stress and interpersonal relationships. Paddock’s physical and mental health had been declining before the shooting. His wealth had diminished, and the 64-year-old struggled with aging, the FBI said.

Trump, who met with victims at a hospital in the days after the massacre, said the FBI “worked very hard on the case” but “they just were unable to find anything.”

“A very sick person, who just, people never saw that coming.” Trump said.

O’Toole now heads the George Mason University forensic science program in Fairfax, Virginia. She said FBI profilers often work like consultants for police departments in major investigations.

She cautioned that while she didn’t know the specific details of the arrangement following the Las Vegas shooting, police may have told the FBI what specific information they wanted agents to investigate and report on.

Las Vegas police have declined to comment on the FBI report. The department closed its investigation in August with the release of a 187-page report — also without identifying a reason for Paddock’s rampage.

In response to a court order in a public records lawsuit, Las Vegas police previously released thousands of pages of police reports, witness statements and hundreds of hours of officer body camera and surveillance video footage.

“People have a hard time understanding motive,” O’Toole said. “They think of money or sex. We’re seeing more and more these motives are personalized like a desire to be infamous or to be internationally known.”

Paddock broke out a window in his 32nd-floor suite at the Mandalay Bay casino-hotel and fired more than 1,000 rounds in 11 minutes into a crowd of music fans below.

He had 23 assault-style weapons, including 14 fitted with rapid-fire “bump stock” devices, which the Trump administration has banned, beginning in March. Authorities said Paddock’s guns had been legally purchased.

Paddock did not leave a manifesto or suicide note, and federal agents believe he had planned to fatally shoot himself after the attack, according to the report.

“Bottom line is he didn’t want people to know” his motive, Rouse said.

Levin said he thinks Paddock wanted most to be remembered.

“People say we don’t know Paddock’s motive,” Levin said. “But I think his motive was to go down in infamy. That’s a motive that’s shared by lots of killers.”

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Ritter reported from Las Vegas. Associated Press writer Michelle L. Price in Las Vegas contributed to this report.