PHOENIX - ABC15 has learned new details about the investigation into an apartment fire that led to the death of a 14-year old girl .
Sources close to the investigation say detectives brought in a specially trained K-9 able to detect accelerants.
The dog was brought to the apartment as investigators searched for clues.
It is unknown if the dog alerted investigators to any suspicious substances.
ABC15 has also learned detectives removed several pieces of charred debris from the apartment for further testing.
At last check, those close to the investigation say the 33-year old mother of the children has improved while hospitalized and able to speak.
Her children ages, 3 months, 3 years and 10 remain in serious condition in a burn unit. The girls have shown signs of improvement according to investigators.
Phoenix police detectives confirmed to ABC15 they released a person of interest in the case, however, that person remains part of the investigation.
"The investigation from the scene, forensics and more interviews will be necessary to ensure the investigation is conducted properly and the correct suspect is brought to justice," Officer James Holmes said in an emailed statement.
The 14-year old female who died has been identified as Rosa Belen Zapien, friends know her as Rosa Ramirez.
Neighbors told ABC15 they heard people arguing in the apartment days before the fire.
"I heard them arguing the night before, it got really bad, police were called too, they were here," said Vincente Gonzales who is one of the neighbors.
Detectives are investigating the fire as possible arson/homicide.
Gonzales, a father of two, grabbed his two daughters when he smelled smoke and said he could barely see trying to escape as the building burned.
"I can't even close my eyes because of the flashbacks from that night that are still going on, especially the part where the girls (victims) were being hauled off, its hard man, it's really hard," said Gonzales while standing outside his apartment Monday sifting through the damage.
"When it was happening and we got out, I just grabbed my girls and I was telling them not to look when we got out cause you could see that whole family was in bad shape, it was all just so bad."