A library was forced to close for two days after venomous spiders were found in the building.
The creatures were spotted at the University of Michigan and have been identified as Mediterranean recluse spiders.
Professor Ann Danielson-Francois, who works at the university, said that the arachnids are venomous but would be unlikely to bite a human in an open space.
She told WJBK-TV: "The Mediterranean recluse is one of these cosmopolitan species, it hitchhikes with people throughout the world. It's been found in 22 states.
"They don't have a huge amount of venom, but they have enough to start necrosis, which is the eating away of the flesh."
Francois adds that she now intends to use the spiders in her lessons.
She said: "I'm going to keep them for my spider biology class so I can teach students how I identify recluses properly."