WASHINGTON — A husband and wife are dead after a domestic murder-suicide Tuesday afternoon at a gas station in Montgomery County, Maryland.

Police say a man shot his wife in a car before he turned the weapon on himself at a gas station in the area of Md. 118 and Md. 355 in Germantown, Maryland, at about 12:20 p.m.

They were in a car when an argument occurred at the gas station. A 16-year-old girl, who police say was a family member, was sent into the store to pay for gas.

The husband and wife stayed in the car and started arguing until the man shot her, police say. He then shot himself.

The woman, who was taken to a hospital in grave condition, later died, police say. Neither the gunman nor the woman has been identified.

Capt. Paul Starks, a spokesman for the Montgomery County Police Department, says the 16-year-old fled when the argument turned violent. The teenager was uninjured and was interviewed by police, Starks says.

An off-duty police officer was at the scene when shots rang out. The officer responded as the gunman turned the weapon on himself. Police recovered the firearm at the scene.

Adrian Jefferson, who witnessed the incident, said he was walking toward the gas station when he heard one shot and saw glass shatter. Jefferson said he saw a woman in the car slumped over, apparently suffering from a shot to the jaw.

Seconds later, when an officer got closer to the car, the man appeared to shoot himself in the head, Jefferson recalled.

“It happened within 30 seconds,” he said.

The incident is being investigated as a domestic murder-suicide, Starks says.

WTOP’s Max Smith contributed to this report.