Angelica Hinojosa, a security guard at the center, said government migration officers are in charge of keys to the locked cells at the site in the northern city of Ciudad Juarez, providing important new insight into how the blaze, one of the worst migrant tragedies in Mexico, became so deadly.
"He wasn't there," Hinojosa, who has worked for security firm Grupo CAMSA at the Ciudad Juarez site for four months, said in an interview at her home on Thursday.
She did not know if the absent migration officer had taken the keys to the men's unit with him or if they had been stored on site, she said.
Mexico's National Migration Institute (INM) and CAMSA did not immediately respond to requests for comment.
Mexican officials on Thursday arrested five people suspected of involvement in the fire, after obtaining arrest orders for three INM officers, two private security officers and the person accused of starting the fire.
The investigation is focusing on why the 68 male migrants held at the center appeared to be left in their cell while the fire burned on Monday night.
A leaked video, which appears to be security footage from within the center, shows men kicking on the bars of a locked door as their cell fills with smoke. Three uniformed people can be seen walking past without trying to open the door. Investigators have said the video is part of the probe.
Hinojosa said the migration official tasked with overseeing the men's unit had left the building around 8:30 p.m. to transfer minors to a government shelter.
The fire began around 9 p.m., prosecutors said.
"He returned when we were already outside; I was with the women," Hinojosa said.
Speaking to reporters on Wednesday, a woman who said she was the sister of the same migration officer confirmed he had worked the night of the fire but left to take minors to a shelter.
The men were evacuated a while after the women had already left their cell, said Hinojosa, who has also spoken to law enforcement about the incident. She could not say exactly how long after.
Several witnesses told Reuters the men were locked inside until rescue teams arrived.
Hinojosa said three migration officers and three private guards typically worked in a single shift at the detention center, but that the workload from high numbers of migrants had increased. She declined to comment when asked about CAMSA's track record on training and safety.
Hinojosa's job was to help watch over the women detainees, she said.
On Monday night, she began to see smoke filter through the bars of the cell and dashed to the migration official who had the key to the women's unit.
That person handed it to Hinojosa, who hurried back to the cell and told the women: "Run."
"I unlocked the gate and got them out," Hinojosa said.
"The only thing I could think of was saving them," she said, her voice cracking. All the women were evacuated safely.
Officials have said they will replace CAMSA's services with federal guards in Chihuahua state, where Ciudad Juarez is located, and flagged concern over whether the company's guards were properly trained.
Hinojosa said she felt concerned she would be blamed unfairly. "We are the support," she said, referring to her role helping migration officers. "I did my job."