(CNN) -- Marie Mills held her 77-year-old father, who had collapsed outside in a Washington street. She screamed for help.
A passerby rushed across the street to bang on the door of a fire station, knowing that firefighters are trained to provide emergency medical help.
But they wouldn't leave the station.
The same thing happened when two more people tried to summon the firefighters for assistance, Mills says.
"We looked across the street at the fire station. There was a firefighter that was actually standing against the fire apparatus," she told CNN affiliate WJLA. "Everybody started trying to wave him over." But the firefighter said he had to be dispatched first.
"I even ran to the curb and said, 'Are you going to help me or let my dad die?'" said Mills.
Later, after an ambulance finally arrived, Cecil Mills died at a hospital. He had suffered an apparent heart attack.
An investigation is under way and, so far, no officials are publicly challenging Mills' version of events.
"It's an outrage," Washington D.C. Mayor Vincent Gray said at a news conference. "I was absolutely furious."
He said he apologized to Marie Mills for "what appeared to be dereliction," CNN affiliate WTTG reported.
"Those who failed to respond as they should -- they will be held accountable, period," Gray vowed.
In comments Thursday to CNN, Gray said he has "taken a lot of time with it."
The investigation is "being done as rapidly as we possibly can," he added.
The D.C. Fire and Emergency Medical Services Department told CNN it is investigating the incident, which took place along the 1300 block of Rhode Island Avenue on Saturday.
"Our duty is to respond to all requests for emergency assistance. If it is determined that proper protocols were not followed at the conclusion of our investigation, then appropriate action will be taken," spokesman Tim Wilson said.
The protocol is in question because, according to the Mills family, those who asked for help at the fire station were told to call 911.
Calls to 911 were placed, but a mix-up with the address delayed an ambulance, said Washington Council member Tommy Wells.
"Two things happened," he said. "One was that no one came out of the fire house to help this gentleman. The other is the ambulance that was dispatched was dispatched to the wrong place. This was a number of fiascos."
Paul Quander, deputy mayor for public safety, said a "very new, probationary employee" was at the facility. The employee's first response should be "to ask a senior person, and we believe that was done," he said, according to CNN affiliate WTTG. "The question now is what did that senior person say? What did that person do? Did they follow protocols and procedures?"
There are no protocols that would prevent fire personnel from helping those in need, Quander told CNN Thursday. "These are people who run into burning buildings. They work in the most hazardous conditions imaginable.
"They cross the streets, they cross highways to get to people. It is understood. It is common. This is why it is so troubling that we did not take the appropriate action in this case. It's right across the street."
"It's hard to get your arms around" the idea that this took place, he added.
Lt. Kellene Davis was the officer in charge of the station at the time of the incident, WTTG reports.
Davis did not respond immediately to an e-mail Thursday from CNN, and a call to a phone number listed for her was not answered.
Cecil Mills, a lifelong Washington resident, worked for the Department of Parks and Recreation.
His daughter, in mourning, had kind words for the mayor's handling of the matter. "I appreciate how seriously he is taking this because it never should have happened," she told WTTG.
Marie Mills wasn't immediately available Thursday when contacted by CNN.
The firefighters' union said the incident simply should never have happened.
"We need to find out why it did occur and make sure it never happens again," said Ed Smith, president of the DC Fire Fighters Association, in a statement reported by WTTG.
He added that on the union's behalf, "I offer Mr. Mills' family a sincere apology."