D.C. police are conducting an internal investigation after it took more than 20 minutes for police to provide backup to a firefighter and a paramedic who were being attacked by a woman with a knife -- despite their repeated pleas for help.

Sources familiar with the investigation told News4 that about 2:44 a.m. Thursday, a firefighter and paramedic responded to a call about a heart attack at an apartment in the 500 block of 59th Street NE.

The D.C. Fire and EMS crew had responded to the same address several times before and the woman at the apartment had at one point threatened them with a knife, according to audio from their first call requesting police backup.

Over a period of more than 20 minutes, the firefighter and paramedic made at least five more pleas for police to come to the scene and reported the woman was coming at them with a knife and trying to stab them.

"We need them here, now!," the firefighter said during his fifth call to 911 at about 3 a.m.

Witnesses said the paramedic was wrestling with the woman trying to protect himself and get the knife from her as the firefighter pleaded with the dispatcher.

When they call for a sixth and final time, the dispatcher tells the firefighters that police are refusing to come.

"Per supervisor, they said officials advise they’re not coming back to that location," the dispatcher said. The firefighter then responds, "Okay, well tell the supervisor she has a knife. She’s threatened to kill us and herself. 1033."

Code 1033 means a firefighter's life is in danger and they are requesting police.

Police units responded after that call, at least 21 minutes after the first call for help. The woman has been charged with assaulting an officer and making threats.

A spokesperson for the Metropolitan Police Department gave News4 the following statement:

"The individual had called 911 approximately a dozen times in the preceding 24 hours. Each time, units from the Metropolitan Police Department or the Fire and Emergency Medical Services Department responded, and in each instance, the individual refused assistance. During the call for service in question, the individual sought FEMS assistance for a medical issue; there was no initial report of any weapon or threats. Once MPD was informed that the individual was making threats and possessed a weapon, MPD units arrived on scene within two minutes. The individual was placed under arrest for assaulting an FEMS employee without further incident. We take the safety of our public safety colleagues very seriously and stand by to assist FEMS whenever they call."

D.C. Fire and EMS, the 911 call center and Mayor Muriel Bowser's office denied News4's requests to be interviewed, saying the incident is under investigation.