A swarm of bees attacked a suburban park in California on Sept. 3 stinging more than 20 people and sending three to the hospital.

The Los Angeles County Fire Department said that the attacked happened in the Cerritos Regional Park, which is about 20 miles south of Los Angeles. The incident reportedly occurred at the tennis court of the park where a family reunion was happening at the time.

Witnesses said that thousands of bees chased hundreds of people and the insects appeared to be drawn to people with long hair.

"It was thousands. It was thousands, like this whole grass area was covered with bees," a park visitor said. "They were chasing hundreds of people."

Ambulances with red lights and sirens were dispatched to the area, but 20 people have been stung by the insects.

The bees left the park within an hour, albeit it was not clear what caused the insects to leave their hives and attack the crowd

Most bee attacks, however, are provoked. People throwing materials at the bees, for instance, can provoke the bees to attack. Loud noise such as that of the lawn mower may also attract these insects. The bees appeared to have come from inside of an underground utility box on the field.

Dispatching supervisor Bernard Peters that the three people who were hospitalized are expected to be fine.

Earlier this year, a swarm of aggressive bees also attacked a California neighborhood. The insects stung several people and were believed to have killed two dogs.

Bees are important pollinators and are considered crucial in the production of fruits and vegetables that consumers eat. The insects are often harmless but perceived threats can set off an attack.

The U.S. Department of Agriculture (USDA) advises running away quickly when bees attack. Swatting the bees or flailing your arms is also discouraged because the insects are attracted to movement. Crushed bees also produce a smell that can attract more bees.

Those who were stung more than 15 times and those allergic to bee stings are urged to seek medical help immediately. About 2 million Americans are believed to have allergies to the venom of stinging bugs and these people are at risk of suffering from life-threatening allergic reactions.

A severe allergic reaction to bee stings called anaphylaxis can be potentially deadly and needs emergency treatment. According to the Mayo Clinic, a small percentage of those who get stung by bees or other insects quickly develop anaphylaxis.

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