It was a chaotic Thursday morning at St. Paul’s Como Park Zoo.

Zookeepers were supposed to double-check all the exhibit doors, but on a busy summer day, no one noticed that one didn’t get locked.

But the gorillas noticed.

Three of them, a group of bachelors, did a little exploring just before the park was to open. Around 9:50 a.m., a zookeeper noticed that Sampson, Jabir and Virgil were missing from the replica jungle in the indoor Gorilla Forest. A keeper radioed a Code 99 — a zoo alert reserved for potentially dangerous animals and situations.

Allison Jungheim, senior zookeeper, said the gorillas had been in the exhibit only 10 minutes before discovering the unlatched door. They pushed through into a secure behind-the-scenes area. Zoo staff knew the apes were safe but because of radio transmission weren’t sure exactly where they were, Jungheim said.

They found the gorillas wandering through the 5-foot-wide hall behind the orangutan and gorilla exhibit. One was bouncing balls, fascinated by the items in a closet filled with activities and food for the animals.

“Any of our animals — if given an opportunity to explore a space not in their own area — are going to check it out,” Jungheim said.

By 10:10 a.m., Jabir and Sampson were back in their habitat. Virgil, the most curious of the trio, was exploring the closet, and it took some coaxing to get him back in his area, Jungheim said.

“No one was ever in danger,” said Matt Reinartz, Como Park Zoo and Conservatory spokesman. “The gorillas were always in a secure, gorilla-proof location. They just happened to be in a location they shouldn’t have been in.”

Six zoo camps were running at the time, but the campers were secure during the incident, Reinartz said. Most visitors only noticed that the opening was delayed until 10:40, from the usual 10 a.m.

The gorillas are big — Virgil is 450 pounds, Jabir is 420 pounds and Sampson is 390 pounds.

Como Zoo’s emergency response procedure depends on the animals involved, Jungheim said. All zookeepers and the general curator are trained to secure animals and the public. In a Code 99, the zoo also calls in its veterinarian at the University of Minnesota. Jungheim brought the tranquilizer, just in case a gorilla acted up.

The jokes flew all day from gorilla fans, and the escapees even got their own Twitter account, @ComoZooGorilla, with the tag: “I’m not stuck in here with you, you’re all stuck in here with me.”

Como’s last great gorilla escape was in 1994. Casey II climbed over a concrete wall and roamed through the zoo for about half an hour as visitors scattered.

The gorilla’s space has been redesigned since then, Reinartz said. Unlike previous designs that let the audience look down at the gorillas, the Como Zoo exhibits now put the gorillas up high so they can feel dominant, he said.

The Gorilla Forest opened in June 2013 with six new residents — plus one familiar face, a 525-pound silverback named Schroeder. The 13,000-square-foot habitat features new interiors and two new outside areas that make up the largest all-mesh gorilla enclosure in North America.

Kalley Ravndalen, 18, walked past the gorilla exhibit Thursday afternoon after hearing about the morning incident.

“It’s a little scary,” said Ravndalen, who is from Lakeville.

Human error is a big factor in animal escapes, Jungheim said.

“Mistakes happen, and that’s the long and short of it,” she said. “But the fact that nobody was hurt, nobody was injured, everyone was safe, all animals are now secure — it makes it all good.”