FILE: Washington D.C. fire, rescue and police work at the scene of an accident. (GARY CAMERON/REUTERS)

A police officer drove an injured colleague to a hospital Tuesday evening after waiting more than 20 minutes for a D.C. ambulance, according to District police and fire officials. The ambulance was canceled before it arrived; the injuries were described as minor.

The incident is the latest in a string of delays in transporting patients to hospitals and comes after the District’s mayor has raised concerns that a recent surge in 911 calls coupled with a growing population is straining the fire department.

As in previous cases, fire officials attributed the delays in getting help to the officer to the District’s entire fleet of 42 ambulances and paramedic units being busy on other calls. In March 2013, another officer waited 30 minutes with a broken leg before an ambulance was finally dispatched from a neighboring jurisdiction.

Earlier this month, a stabbing victim in Southeast waited nearly a half hour for an ambulance, and in another case, dispatchers failed to send the closest ambulance to a child who was choking in Northwest because a computer did not show the unit as available for a call. That incident is being blamed on a new computer tablet used to keep track of fire engines and ambulances.

In that case, it took seven minutes to get help to the boy, who remains in critical condition, according to fire department officials. That time is still within accepted parameters, but officials note that had the system worked properly, paramedics would have arrived sooner.

A fire department spokesman said officials are still investigating why the tablet didn’t show the location of the ambulance. Both the incidents with the choking child and the injured officer were first reported by WTTG (Fox 5).

Gwendolyn Crump, the D.C. police department's chief spokeswoman, said the officer was injured while making an arrest shortly before 6:30 p.m. on Tuesday in the 2300 block of Fourth Street Northeast. “We are aware that a request for an ambulance was made,” Crump said.

A D.C. fire department spokesman said an ambulance was requested at 6:40 p.m. but that all were busy. That was confirmed by Wanda Gattison, spokeswoman for the Office of Unified Communications, which runs the District’s 911 center

The fire spokeswoman said the officer was being treated by a firefighter who arrived within minutes but that ambulances were tied up elsewhere. He said a supervisor arrived at the scene at 7:09 p.m., but by that time the police officer had been taken to the hospital in a squad car.