We have just commemorated the 13th anniversary of the Sept. 11 terrorist attacks, yet many people may not realize that September marks another such tragic milestone. It’s been one year since an Al Qaeda-linked terrorist group based in Somalia attacked a shopping mall in Nairobi, Kenya, leaving 71 people dead and hundreds wounded.

A new documentary airing on HBO, “Terror at the Mall,” takes a harrowing look inside the siege by Al-Shabaab militants at the upscale shopping center known as Westgate, which lasted for a staggering 49 hours before the situation was brought grimly under control.

Directed by Dan Reed, who also helmed HBO’s 2009 Emmy-nominated “Terror in Mumbai” and 2003’s BAFTA-nominated “Terror in Moscow,” the film recalls the horror of the attack and the courage of ordinary citizens who were caught in the middle of a murderous rampage — people who had been going about their daily lives on a typical weekend afternoon.

“Somehow, it really got to me. It’s a universal location meant for all of us, and this is a mall that looks like any other in Europe or America,” Reed said in a phone interview. “It was a Saturday lunchtime, people doing what they’re doing, walking around with their kids, shopping. There was a generic significance that made it chilling.”

The London-based director started working on the film right after the attack and made five trips to Kenya, using footage from more than 100 security cameras, which recorded hours of surveillance video, along with extensive photographs taken during the siege. He tracked down and interviewed many of the survivors and some of the rescuers, who reflected on what took place, why it happened the way it did and how their lives have changed in the interim.

“We recorded 82 interviews but met 150 people involved,” Reed said. “My film tries to create an account of what it was like to experience the attack. It was designed to be a claustrophobic experience. These are important events for us to understand and that’s why I try to have material that gives you an incredible inside view and allows you to piece together a complex event that unfolds in a rapidly changing scenario.”

The terror began out of nowhere at 12:30 p.m. when a shopper passed through a security check to enter the mall, heard a loud explosion, which turned out to be a grenade, and then saw the guard who had been searching him fall to the ground. Witnesses recalled that gunfire quickly erupted and footage shows patrons in a restaurant diving for cover or being knocked to the ground.

As the security camera videos dispassionately reveal, chaos engulfed the mall, with frightened shoppers running for their lives, unsure of the origin of the attack. Many tried to find hiding places within the shopping center, including under display tables, while literally hundreds of terrified people fled into a giant two-story supermarket, Nakumatt.

Four terrorists were responsible for the rampage. Two of the gunmen made their way toward the supermarket while two others headed for the mall’s rooftop, where a children’s cooking competition was under way.

A 15-year-old girl who was shot in the stomach, thigh and foot clearly recalled one of the terrorists’ chilling battle cries. “The only thing he said was that we are here to kill. You killed our people in Somalia. We normally don’t kill women and children but you kill ours in Somalia and so we are here to take revenge.”

In October 2011, Kenya had marched into Somalia to combat Islamist jihadists who had been kidnapping Westerners in its border region, but Reed said there was little evidence of Kenyan civil rights abuses of Somalis during the incursion.

Meanwhile, inside the supermarket, 20 people had hidden behind the meat counter when the terrorists started shooting them, letting some go who said they were Muslim. One woman who was protecting her young daughter and son was shot through the pelvis.

Outside the mall, 45 minutes after the shooting began, Kenyan security forces tried to decide how to proceed, as time ticked away for the wounded waiting to be rescued.

“We laid there for very long time,” recalled one woman who was trapped on the upper level. “You would expect to see a lot of armed soldiers coming up the ramp. Maybe that’s what we were expecting, but that didn’t happen.”

It’s painful to watch the injured struggling, but as the security forces dawdled, a handful of plainclothes police and civilians decided to act — seven in total — going into the mall and rescuing seriously wounded people who had been clinging to life amid the carnage.

The obvious question arises. Why were Kenyan forces so impotent in stopping the attack, preventing further carnage and rescuing the victims?

“It’s not that easy to compare the response to what the American or British response would be,” Reed said. “The institutions in Kenya, sadly, are very dysfunctional. The military and the police are not oriented toward saving lives. The long and short of it is the priority was not to save lives — people were basically covering their asses not wanting to take a risk,” he said of the security forces.

Some of the most graphic and gut-wrenching security camera footage shows one man being repeatedly shot at close range just inside the mall’s entrance, seemingly as he is about to escape. He had been a driver for an American charity. Reed interviewed the man’s daughter.

“He didn’t realize there were two pairs of gunmen. He thought the terrorists were behind him and mistook the two other armed men for cops before they casually shot him at point blank range,” said Reed.

Part of the process in producing the documentary involved much forensic work, analyzing imagery and investigating obscure elements that can lead to useful conclusions.

“As we’re confronted with these hugely impactful events, it’s important to understand the cruelty and brutality but also the astonishing courage and selflessness of people who worked together to survive,” Reed said.

He points with admiration to three women who were there with their children. “It gave them the ability to focus on survival that they wouldn’t have had if they had been there alone,” Reed said. “It supercharged their senses and really helped them to survive. That’s something that gives me hope, a redeeming side of the story. I find that a reason to be optimistic about the human race.”

This may also be a fitting conclusion to the horrifying story of terror at the mall. On Sept. 1, Ahmed Godane, the leader of Al-Shabaab and the apparent mastermind of the Westgate attack, was killed in a targeted U.S. military airstrike in Somalia.

(“Terror at the Mall” premiered Sept. 15 on HBO and has a number of rebroadcast dates lined up on the pay-cable network, including Sept. 18, 21, 23 and 27, with additional screenings on tap on HBO2. The program will also run on CNN Sept. 26 at 9 and 10:30 p.m. ET.)