A 13-year-old boy from New York state was in critical condition in a Boston hospital Thursday morning after he fell out of a moving tour bus Wednesday afternoon in the Katama section of Edgartown.

Edgartown police were called to the scene of the accident on Herring Creek Road near Atlantic Drive at about 4 p.m. Wednesday. The victim, whose name has not been released, was lying in the roadway, police said, and had apparently fallen from an emergency exit window in the tour bus.

The boy was with a group visiting from Camp Greylock, a residential camp for boys in western Massachusetts.

“It was definitely one of the emergency exit windows,” Sgt. Joel DeRoche said, describing the accident. “Somehow the window unlatched. In my initial approach to the scene, the window was slightly ajar.”

Edgartown EMS took the injured boy by ambulance to the Martha’s Vineyard Hospital, where he was soon flown by Medflight helicopter to Children’s Hospital in Boston. Sergeant DeRoche said as of Thursday morning his condition remained critical.

A witness told police the bus was traveling at a normal rate of speed at the time of the accident.

“It doesn’t appear speed was a factor,” Sergeant DeRoche said.

Police are investigating the incident, including whether the emergency window malfunctioned, opened accidentally, or opened for some other reason.

The vehicle is owned by Island Transport Inc, a well-known tour bus company based in Oak Bluffs, police said. The bus, formerly used as a school bus, is 16 years old.