A swarm of bees from a hive estimated to hold 800,000 attacked four landscapers Wednesday morning in southern Arizona, leaving one dead and another critically injured.

The men had been mowing grass and weeding for a 90-year-old homeowner in Douglas, Ariz., when the insects emerged from a 3-by-8-foot hive in an attic and attacked the crew, the Douglas Fire Department said. One man died at a nearby hospital. “A witness said his face and neck were covered with bees,” Capt. Ray Luzania told Tucson.com.

The other man, who was stung more than 100 times, was treated at the hospital and released.

Two other workers who were stung refused treatment and a neighbor, who was also stung, drove to the hospital.

Douglas Fire Chief Mario Novoa told USA Today that because there aren’t many honey bees left in the area, “we treat them all as Africanized [killer] bees.”

None of the victims have been identified, and it’s still not clear what provoked the bees or how many were involved, Novoa said. But if they were killer bees, it wouldn’t take much to excite them — even small noises or vibrations have been known to do it.

In the past, such attacks have been described as a scene straight from a horror film, with swarms so large the sky turns dark. The roar fills victims’ ears. Bees clog their mouths and nostrils when they try to breathe. And hundreds — or thousands — of stingers akin to hypodermic needles pump poison into their skin. The smell of honeybee venom has been compared to bananas.

But it’s not typically the venom from killer bees that kills; it’s the number of stings, May Berenbaum, a professor at the University of Illinois’s Department of Entomology, told CBS News last year.

With killer bees, “the venom is not more toxic,” she said. However, when killer bees are disturbed, “they are more likely to pursue the source of disturbance more consistently.

“Bee venom is a cocktail of biologically active components that are designed to inflict pain. The honey bee stings only defensively — they don’t try to kill, they try to educate.”

After Wednesday’s attack, firefighters in bee suits surrounded the home.

The exterminator who was called to kill the bees estimated their colony had been growing for at least a decade. He said the honeycomb was so compressed, it was pushing them out. They had started building another nest nearby.

“They were dropping down at me even before I started approaching it,” exterminator Jesus Corella told Tucson News Now. “That was before I started spraying. They were dive-bombing me and that’s a sign to back off, back way off.”

The hive filled a 55-gallon drum after extermination, Novoa said.

Berenbaum said there are about 40 fatal attacks each year.

In addition, about 2 million Americans are allergic to bee stings, according to Boston Children’s Hospital. A sting can cause anaphylactic shock for those who are allergic.

It’s not known whether Wednesday’s victims had bee allergies.

The fire department in Douglas, which is about 120 miles southeast of Tucson, gets a couple of calls a week about bees, Novoa said. So far, none of them have compared to Wednesday’s attack.