The home features two stories above street level. Firefighters Vincent Perez and Anthony Valerio from San Francisco's Engine 26 went through the front door.
Prez and Valerio told dispatchers the fire appeared to be small, according to union president Tom O'Connor.
"(They) couldn't find the fire," O'Connor told ABC7. "There was still a lot of visibility and then, all of a sudden, all the smoke just filled the entire house. There was no visibility."
By then, the white smoke changed to a thick black with flames enveloping one floor of the home, which faces toward Glen Canyon Park.
"Then we didn't hear from the crew of Engine 26 again," O'Connor said.
The fire, according to O'Connor, appeared to come from the floor below the two men.
"You're coming in on top of the fire, which is exactly where you don't want to be with all the heat coming at you," O'Connor said.
Other firefighters were attempting to ventilate the house by cutting holes in the roof and ceiling.
Firefighters wear a device called a "pass alarm," which turns on every time a firefighter turns on their airtank.
"If you stop moving for 15 seconds, the alarm will go off until you move again," O'Connor said. "This alarm went off, which makes firefighters know both (Perez) and Valerio were down."
Other firefighters began to move through the black smoke toward the sound of the alarm. O'Connor describes what likely happened next.
"They (the firefighters) went through the smoke. They saw a flashlight aiming at the ceiling and even having that indication, they went towards it," O'Connor said. "They still couldn't see...the smoke was so dense, they couldn't see where they were going and they tripped over one of the guys. That's how they found them."
They radioed their discovery of both men back to dispatchers.
"We have a recovery!" one firefighter radioed. "Send two medic units -- you copy?"
Firefighters then called truck companies stationed outside the house.
"Companies operating on the fire...ground outside right now. I need two companies outside now," a firefighter radioed.
Radio dispatches caught the chilling moments a first responder announced the transport of both down firefighters.
"We have two critical firefighters," the first responder called out, "I am transporting one critical...third degree burns. Both are in full arrest. ETA is about 10 minutes."
Investigators are trying to figure out if the intense heat from the fire was caused by a "flashover," or a sudden ignition of objects in a room due to the increased levels of oxygen.
Both firefighters were wearing protective gear, but fire officials say the gear is only designed to protect at temperatures between 500 and 700 degrees. Officials say the temperature inside the room where the fire was located could have been well over 1,000 degrees.