The National Zoo is looking into an incident involving chlorine that caused a "loud boom" on Thursday morning. No animals or people were injured, and the Zoo will be open for visitors on Friday. (WUSA 9)

The National Zoo and fire officials are looking into an apparent chlorine accident along the outdoor American Trail exhibit Thursday morning that created a “loud boom,” but caused no fire.

“There was an incident related to chlorine on American Trail, near the otters and beavers,” said zoo spokeswoman Devin Murphy. “There was a loud boom. No fire. No smoke.”

“No humans were hurt,” she said. “No animals were hurt. We cleared the area and D.C. fire is on scene and they’re investigating.”

Another spokeswoman, Pamela Baker-Masson, said the incident happened at 10:55 a.m. inside a small building that houses machinery that supports the beaver and otter exhibits. Chlorine is used in the water filtration system for the exhibits, she said.

A contract employee was inside the building. He smelled chlorine, and left. Then there was the boom. The fire department was summoned. Baker-Masson said she did not know what work the contractor the contractor was doing in the building.

There was no gas, she said. The worker was examined by medics. “He is fine,” she said. The affected area was closed to zoo staff until about 2 p.m. Keepers reported that all animals were accounted for and appeared to be in good health.

American Trail winds among wolves, otters, beavers and other North American animals.

Fire department spokesman Timothy J. Wilson said officials suspect the accident might have stemmed from the improper mixing of chemicals.

Meanwhile, life elsewhere at zoo went on.

Zoo-goers strolled past the fire trucks. Some asked what had happened, others wondered about a fire hose, and a few posed for selfies with firefighters.

“I just don’t know how something like that happens,” said Lauren Merritt, 29, of Bellingham, Wash., who was visiting the zoo with her family, including three small children. “I’m just glad everything seems to be okay and it wasn’t any worse than it was.”

“Honestly, my kids are just as excited to see all the police and fire trucks as they are to see the animals,” she added. “It’s kind of making our visit extra interesting.”

A North American River Otter is seen in this file photo. (Katherine Frey/The Washington Post)

After driving from New Jersey, Melanie Delgado and her boyfriend, Mateo Cuecha, said their first stop on their spring break trip in the District was the zoo.

“We had no idea what was going on,” said Delgado, 19, an economics major at New York University. “We were diverted and saw all the fire department trucks. We thought an accident happened or something.”

Delgado, who said they’re staying only for a few days, looked over at the trail. “It sucks that it is closed today,” she said.

One 14-year-old visiting from a Texas school said he heard a little noise Thursday morning. Soon after, fire trucks were rushing in.

“They say it’s safe,” said Anna Galvan, a school counselor from Corpus Christi. “So I believe them.”

Peter Hermann contributed to this report.