February 22, 2020 1:16 PM EST

Two firefighters, one police officer and two civilians were hospitalized after being stung by a swarm of up to 40,000 bees in Pasadena, Calif., on Thursday. The swarm occurred outside of the Howard Johnson Hotel across the street from Pasadena City College.

Pasadena Fire Department spokeswoman Lisa Derderian tells TIME that the department was the first to arrive to treat a civilian, unaware of the size of the swarm. The first firefighter on the scene received up to 15 to 20 bee stings. Derderian said the bright yellow on their uniforms “didn’t help” because bees are attracted to the bright color; the bees then attacked the firefighter’s face and hands. Once the fire department understood the situation, backup began arriving in bee protective clothing.

Because the swarm occurred across the street from Pasadena City College, there were numerous bystanders. On top of the five people who went to the hospital, other were stung but chose not to get treatment.

The fire department called the police to help set up traffic control, so they could close part of the street to address the swarm. Derderian said the first policeman to arrive didn’t realize the severity of the situation and also was stung the moment he got out of the car.

Derderian said that officials don’t know how the bees ended up in Pasadena but they could have been in the area for a long time and just been laying dormant. She said it was unknown what caused the swarm, but that plenty of different disturbances could have set it off: a passing truck shaking the building, someone walking by smoking or a certain scent, for example. She said they were Africanized bees, which are rare for the area and known for being aggressive and attacking in numbers.

“I’ve been with the Pasadena Fire Department for over 18 years and I’ve never seen a swarm of bees of this magnitude,” Derderian said.

Derderian said officials found about six feet of honey comb at the top story of the Howard Johnson Hotel, indicating there were tens of thousands of bees that been there for several months.

She explains that officials used CO2 and foam to mitigate the immediate threat to public safety — especially given how deadly bee stings can be to those allergic — but she said they believe the bee hive is still within the walls of the hotel. She says the hotel needs to get a professional service to remove the hive.

All five individuals were released from the hospital later on Thursday night.

Write to Madeleine Carlisle at madeleine.carlisle@time.com.

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