Bees descended on Encino, California on Monday, sending one resident and a volunteer police officer to the hospital, according to CBS News.
"There is like a cloud of hundreds of bees over there," Izak Kharrazi of local bee removal company All Valley Honey & Bee told CBS News Los Angeles. "I said, 'Oh boy'... that's why right away I put my gear on because I can see them. They're mad as could be."
One adult man suffered "multiple bee stings" and was sent to the hospital to recover. Another man, a local volunteer police officer, was caught on video trying to help a motorist when the bees began swarming him.
In the video, captured by CBS Los Angeles' helicopter, the man can be seen talking to a motorist when he begins swatting at bees that have begun swarming. As the bees continue to sting him he wanders away from the car, continuing to swing at the insects. He eventually gets spun around and topples to the ground.
As he recovers it becomes apparent he has been injured by the fall as blood is left on the sidewalk where he fell. Others helped evacuate him from the swarm, after which he was taken to a local hospital where he was treated for "dozens of bee stings."
Police said the volunteer is in stable condition. The condition of the second man injured is currently unknown.
A local resident reportedly told police he also saw a UPS delivery driver attacked by the bees.
"He started saying, 'Run, bees!' And I got hit right in the lip," Jerry Spotts told CBS Los Angeles.
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The bee removal company called out to respond to the swarm told the outlet it believes the insects originated from a hive connected to a house in the neighbourhood.
The company sprayed the hive in an attempt to eradicate the swarm. The local fire department issued a warning to residents to "remain indoors and close all windows/doors."
"This is not a normal behaviour for these bees to be this angry, to want to kill you," Mr Kharrazi said. "This is not OK."
Typically bees swarm when they are preparing to move to a new location or when they are in the process of rearing a new queen, according to the University of California, Riverside.
"Swarming bees are somewhere in the process of rearing new queen, departing colony with old queen, clustering on an intermediate site and looking for a good cavity, flying to it, or moving into it," the university writes.
Bee swarms are typically not dangerous to humans unless humans disrupt or otherwise threaten them, according to Purdue University. Swarms can occur at any time of the year but typically happen in the late spring or early summer.
It is unclear why the swarm in California became agitated and started attacking residents.