A Metrobus driver involved in a multivehicle crash Sunday remains hospitalized and the collision is under investigation, Metro officials said Monday.

The bus operator, who has been working for Metro for about nine years, is being treated for injuries not considered life-threatening, Metro spokeswoman Sherri Ly said. The crash occurred Sunday afternoon on Rockville Pike.

Three other people were injured, including a person who Montgomery County Fire and Rescue Service officials said Sunday was in serious condition with potentially life-threatening injuries. The Montgomery County Police Department did not return requests for comment Monday, and an update of the person’s condition was not known.

The crash happened about 12:20 p.m. as a Metrobus carrying no passengers traveled south on Rockville Pike; the driver lost control and swerved onto Congressional Lane. The bus struck a Honda Passport and landed on the front of the SUV. That collision led to crashes involving three other vehicles, police said.

Ly said Monday that the transit agency had ordered the bus driver to undergo drug and alcohol testing as required by Metro after crashes. Those results were not available Monday. Metro did not identify the bus driver.

“In addition, Metro will conduct a full safety investigation looking at any contributing factors and will continue to fully cooperate with the investigation by Montgomery County Police,” Ly said.

The bus was out of service at the time and had been serving as a shuttle, running from the Rockville Station, which is temporarily closed for a canopy construction project, to other stations on the Red Line, Ly said.