WASHINGTON — 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.