Vehicles pass Pulse nightclub on June 21, 2016, in Orlando. (Gerardo Mora/Getty Images)

Surveillance footage from inside an Orlando club the night of a rampage there shows Omar Mateen firing at people, stopping and then shooting again those who were already wounded, said officials who have viewed the evidence.

The video suggests that “he was making sure anybody who was shot was dead,” said one official, who like others spoke on the condition of anonymity because the investigation is ongoing.

The footage corroborates witness accounts that Mateen quickly emptied a 30-round magazine and then reloaded, proceeding to fire another 30 rounds into the wounded sprawled in front of him.

The video of the attack has become a key piece of evidence as officials try to determine what happened in the early morning hours of June 12. The FBI declined to comment.

Witness accounts indicate that Mateen killed most of his victims during the initial minutes of the massacre. He was using a Sig Sauer MCX rifle firing .223-caliber ammunition at a muzzle velocity of about 3,200 feet per second. And he was a trained shooter from his time as a student at a police academy.

People who knew Orlando nightclub shooter Omar Mateen describe him as a man who had many demons and potentially led a double life. (Erin Patrick O'Connor,Jayne Orenstein,Thomas LeGro/The Washington Post)

On the Saturday night before the shooting, before leaving his home, Mateen told his wife he was going to see an old friend nicknamed Nemo, said officials and others familiar with the case. He then drove to Orlando. Officials said that Mateen got an entry wristband for Pulse nightclub, went inside and then left. He later returned and began his rampage.

In recent days, the Orlando Police Department has faced tough questions about its officers’ actions during their hours-long standoff with Mateen at Pulse. Some inside the club have wondered, for example, whether police should have moved more quickly to confront the gunman, and whether any of those who were gravely wounded inside the club died because rescuers were unable to reach them.

Mateen told authorities that he had a vehicle outside with bombs and a vest like the kind “used in France.” Officials on the scene took the threat of explosives seriously.

During a news conference this week, authorities repeatedly defended their response at the club. The officers who arrived that night “should not be second-guessed,” U.S. Attorney A. Lee Bentley III said. “They performed valiantly during those early morning hours. Lives were saved because of their heroic work.”

Investigators are still filling in a timeline of the evening’s events, which were often marked by confusion about the shooter’s intentions.

Casualties of America’s worst mass shooting

An officer working security off-duty at the club first exchanged gunfire with Mateen shortly after 2 a.m., and by 2:08 a.m., more officers were rushing in for another confrontation.

One officer who arrived at the scene said that he and others went into the club but did not see Mateen, who appeared to be holed up in the restroom area. The officers were told to hold their positions, aiming their rifles toward the restrooms, said Officer Brandon Cornwell, and they waited “15 or 20 minutes — could’ve been longer” for a SWAT team to arrive.

Officials have said that at 2:35 a.m., Mateen called 911 from a restroom where he had apparently taken hostages.

Authorities tried to take whatever victims they could out of the club — from a patio, the dance floor and dressing rooms — and began negotiating with Mateen, officials have said. Those talks broke down just after 5 a.m., about the time that Mateen threatened to strap hostages with explosives and send them from the restroom area out into the club, authorities have said.

The final confrontation was perhaps the most chaotic. Authorities detonated an explosive charge to breach the wall to the restrooms, but it did not create an opening as intended, Mark Canty, the Orlando Police Department’s SWAT commander, has said. Police then used an armored vehicle to ram a hole, which turned out to be between two of the club’s restrooms, he said.

Canty said that Mateen was in the north of those restrooms and that officers created another hole in the south restroom so that hostages could escape. Mateen, he said, soon emerged from the other hole — between the two restrooms — where he was fatally shot in a final gun battle with police.

Orlando Police Chief John W. Mina said at a news conference Monday that he thinks “there was this misconception that we didn’t do anything for three hours,” and he emphasized that police officers were continuing to work, an effort that included heading back into the club to rescue people. He declined to say whether officers had inadvertently wounded patrons in the club during the encounter but said Mateen bore responsibility for what transpired.

“That’s all part of the investigation, but here’s what I will tell you: Those killings are on the suspect and the suspect alone, in my mind,” Mina said.

Dorian Wayne, who was in the club that night, said that he thinks police “could have run in and shot” Mateen, but that he is sympathetic to the tough decisions law enforcement officials had to make.

“I feel for the people who were in there and didn’t get the proper help that they needed,” Wayne said. “But at the same time, I understand the cops — they’re human.”

Thomas Gibbons-Neff contributed to this report.

An earlier version of this article incorrectly said the date of the shooting was June 19. It was June 12.