A swarm of bees attacked a crowd attending a school graduation ceremony in Pasadena, California, Wednesday.
Three adults and a child had to be treated after being stung at Pasadena High School at around 6:30 p.m. (9:30 p.m. ET).
The honey bees attacked to defend a hive they had established inside a sprinkler vault on the campus, and firefighters used foam to destroy the nest.
"If you're out in a public space and you notice bees flying up against your head, bouncing off your head, turn around and go in the other direction," said beekeeper David Williams, who responded to the scene in Pasadena.
"It's usually an indication that you've entered their defensive space and they're trying to warn you to get back," Williams added.
All four victims were treated at the scene, then released after it was established they had suffered no allergic reactions.