Police investigate after a man was shot and killed by another man with a crossbow at a Toronto library branch. (CBC)

Police have made an arrest in the crossbow slaying of a man in a library in Toronto's east end.

The attack happened just after 4 p.m. ET Thursday at the Main Street Library, south of Gerrard Street East, police said.

The man, believed to be in his 40s, was pronounced dead at the scene after being struck down by a crossbow projectile known as a bolt, police told CBC News. A bolt is shorter than an arrow but can be much heavier.

Witnesses told CBC's Jermaine Hylton they saw a man approach the victim, who was sitting on a bench in the library, and then pepper-spray him. He was then shot, either in the face or the back, witnesses said.

Police officers saw a man run north on Main Street after the shooting. He jumped into a U-Haul rental truck and drove off.

Police tracked the truck, found it in the east-end Scarborough district and arrested the driver.

"It's still unclear exactly what sparked this incident. The motivation behind this killing is certainly a major factor here," said Const. Tony Vella.

'He was calm'

Linus Smith said she was sitting in a restaurant across the street when a man came out of the library with something in his hand, got into a U-Haul truck and drove off. Smith said another man came out of the library and took down the licence number of the truck as it left.

The driver "came out of the library, he was calm. He went into the U-Haul and he drove off," she said. "He didn't speed off or anything, he just drove."

Smith said a librarian told her a man "got hit" inside the library.

Main Street was closed between Gerrard Street East and Swanwick Avenue while police investigated.

Toronto Public Library officials said in a statement on Twitter that the branch was "closed until further notice. Our thoughts are with the families. Our staff are all safe."

With files from The Canadian Press