The D.C. Fire and Emergency Medical Services Department is investigating the death of a five-month-old boy who died Thursday afternoon after the delay of an ambulance. (WUSA9)

The 911 dispatcher who took a frantic call Thursday about a D.C. infant in distress did something emergency personnel have had to do with unusual frequency this summer: call neighboring Maryland for assistance.

It was the 58th time since Aug. 1 that the District requested help because it had run out of ambulances, Prince George’s County fire department spokesman Mark Brady said. Typically, the city asks for aid about five to eight times a month.

On Thursday, no cavalry was available, Brady said. Every nearby ambulance in Prince George’s was already answering a call.

D.C. officials said a crew of firefighters — including one trained in advanced life support — rushed to the apartment building in the 4600 block of Hillside Road SE and tried to save the infant, whose name and cause of death were not released.

They worked on him for several minutes, then lifted his tiny body onto a firetruck, fire officials said, cradling him as they took him to an ambulance that was finishing another call more than a mile away. The baby was taken to Children’s National Medical Center, where he was pronounced dead.

Whether the immediate arrival of an ambulance and paramedics could have saved him remained unclear. But for the second time in six months, Mayor Muriel E. Bowser (D) and the city’s embattled Fire and Emergency Medical Services Department are launching an investigation into the emergency transport of an ill or injured child who ultimately died.

“We’ll take a look at what other units were responding to,” said fire department spokesman Timothy Wilson. “We’ll look at how they were all in service and how they were all transporting or dealing with patients.”

Bowser, too, pledged a full examination of what happened, even as she defended the response time of Engine 30, the first fire truck on the scene.

The crew of that engine arrived less than four minutes after it was dispatched. But city officials said it took more than two minutes to dispatch the firetruck after the 911 call came in — considerably longer than the 90 seconds that is considered a national standard.

In March, days after the death of a toddler who choked on a grape in his family’s Northwest Washington home, Bowser said that the city would put an additional 10 ambulances on city streets during peak hours in an effort to ensure adequate service.

But despite record numbers of 911 calls this summer — 579 a day, on average, in August — City Administrator Rashad M. Young acknowledged Friday that the fire department has met that goal rarely, most recently in mid-July.

In the incident involving the choking toddler, D.C. paramedics who were located blocks from the home were not dispatched because of a glitch with new software. Instead, a unit located about a mile away was sent, and it took almost seven minutes to arrive.

That same month, an injured police officer waited more than 20 minutes for an ambulance before a colleague drove him to a hospital, and a stabbing victim waited almost 30 minutes for an ambulance.

On Thursday, fire officials said, all 39 ambulances and medic units that were in service were busy at 3:08 p.m. when the call came in for the child in distress.

As the mercury climbed above 90 degrees for the 48th time this summer, firefighters drove to the child’s home, in a rundown apartment building with no air conditioning and poor circulation, where most pay rent with the help of federal Section 8 housing vouchers.

“It happened so fast,” said Cassandra Thompson, the infant’s aunt, who was in the apartment when firefighters arrived.

She spoke briefly to a reporter Friday, but said that the infant’s mother declined a request for an interview.

Young said the administration has been unable to keep up with the time-consuming maintenance required to keep 10 additional ambulances from its aging fleet available.

But he said the stopgap measure of relying on those often older ambulances will be over soon. The District is about to sign a lease for 14 ambulances to pump up its force, he said, and plans to bring 46 new ambulances into service over the next 12 months.

“It’s this big cycle,” Young said of the department’s long-standing maintenance backlog and equipment shortage. “If you’re already behind the eight ball, you can’t change it all at the same time. We’re going to get new equipment, but it is a phased approach.”

Ed Smith, head of the firefighters union, said the union and Bowser’s administration are on the same page when it comes to the need for more equipment. He said part of the challenge for the city is people who “abuse” 911, calling for minor injuries or for transport to the hospital.

“When we get calls for stubbed toes or a headache, it’s really not a true emergency,” Smith said. “As I talk to firefighters in the street, that’s their highest frustration.”