The San Marcos fire marshal said the process will take time because every piece of evidence, every item in that building will be touched by investigators' hands.
"You will notice there are cranes. There are heavy equipment. There are command buses. There are tents," San Marcos Fire Marshal Kelly Kistner said.
Investigators say it's too early to say whether they suspect arson
and explosive investigations.
To find where the fire started, investigators look at four different types of information: witness statements, fire patterns, fire dynamics and arc-mapping.
"Which is an analysis of the electrical system in the building to show where different parts of the electrical system failed at what time to get a timeline to see potentially where that fire started based on that information," said Paul Claflin, ATF's Western National Response Team supervisor.
When investigators look at fire patterns, they're looking at the mass loss of the materials.
"So if you have a 2x4 in a wall that is more consumed in one place than another, that's either caused by time or intensity. Either the fire was very intense there for a short period of time to cause that damage or it burned there for a longer period of time," Claflin explained. "So everything that's in that building, that's been damaged by fire, we'll look at from a standpoint of mass loss, looking at time and intensity."
Claflin's team brings together bomb techs, scientists, engineers, K-9s and forensic auditors.
The ATF created their National Response Team in 1978. Since then, they've worked close to 820 cases around the country, including the recent serial bombings in the Austin-area.
One of the teams also worked on the investigation into the fertilizer plant explosion in the city of West, Texas back in 2013.
Local investigators will be trying to piece together a cause as well. Kistner said that the investigation will be happening at the scene at least through Friday. He added that while investigators may not be on the scene then, the actual investigation may continue for months afterward.
"It's a slow process, it is a big scene that we are working with," Kistner said.
Joe Loughran knows this process takes more time than many in the general public expect. He is a fire and arson instructor at Austin Community College. He also worked in the past as an arson investigator for the Austin Fire Department.
"Give them the time to do the job they need, to come up with the conclusions, because sometimes it does take time, but in order to be effective and be accurate you need to give them the time to do that," he said.
Loughran explained that investigations where a death is involved become even more complicated because, in addition to investigating the cause of the fire, there will likely be separate investigations into the cause of each death. The more the fire has destroyed buildings, the more difficult that process becomes.
"If that fire goes on for a long period of time, a lot of times that evidence is destroyed," he said.
Loughran added that investigators will stay alert for signs of possible arson, such as disagreements between people at the building, devices designed to set fire, or things designed to accelerate fires. With multi-level apartment fires, Loughran explained that investigators will look at each floor to try and narrow down where the fire could have started.
"And because of the fires, we have ceilings collapse, we have building materials collapse into each other, a lot of times there's like de-layering of the scene where you're picking through a scene one layer at a time, trying to get down to the layer where the actual fire started," Loughran said.
He noted that older buildings without updates to fire safety technology can decrease the likelihood that people escape safely. Sprinklers, for example, can put out a fire or keep it at bay, Loughran said.
"Without the sprinklers, the time that people have to get out is lessened, and also possibly the time they're alerted to the fire," he said.
The complex which burned in San Marcos was nearly 50 years old, authorities said in a press conference Monday. The building did not have sprinklers because at the time it was built that was not a requirement.
Fire safety requirements like these aren't often on renters' radar, explained Juliana Gonzales, the executive director of the Austin Tenants' Council.
"Unfortunately most of the time renters are so constrained by the chances of finding available affordable housing that they're not able to put a lot of attention into finding housing that has the most recent fire prevention systems," said Gonzales said. "That's unfortunately the situation we're experiencing, in many of these Texas renters markets."
She added that there may be ways to prevent these older apartment complexes from going up in flames.
"Cities could set a higher standard for higher safety, even for aging properties," she said.
Gonzales noted that a recent move by the city of Austin requires carbon monoxide alarms in all homes that have far or fuel-burning appliances. She suggested that something similar could be done to increase fire safety standards, though she acknowledged that doing so would require money.
The Austin Apartment Association also released a statement to KXAN:
"There is very little we can say at this juncture to make sense of this tragic loss of life and property. Apartment owners and managers consider resident safety and security paramount, and no matter how much a property tries to prepare for these situations it does not lessen the heartbreak we feel for all those affected. We have great respect for the San Marcos Fire Department the fire safety professionals that will fully investigate this situation, and we are prepared to educate apartment communities about their findings for the safety and betterment of apartment residents in the Austin area and beyond."