CIUDAD JUAREZ, Mexico >> Mexican authorities said Wednesday that eight employees or officials are being investigated for possible misconduct at a migrant detention center where a fire killed 39 detained men.

Anger and frustration in the northern border city of Ciudad Juarez boiled over as hundreds of migrants walked to a U.S. border gate hoping to make a mass crossing.

Mexican officials appeared to place blame for the deaths in the fire late Monday largely on private, subcontracted security guards at the detention center in Ciudad Juarez, across the border from El Paso, Texas. Video showed guards hurrying away from the smoky fire apparently without trying to free detainees.

No charges were announced, but authorities said they would seek at least four arrest warrants later in the day, including one for a migrant who was part of what they described as a small group that started the fire. They said a migrant also damaged a security camera inside the cell where the fire occurred.

Five of those under investigation for possible misconduct are private security guards, two are federal immigration agents and one is a Chihuahua state officer, federal Public Safety Secretary Rosa Icela Rodríguez. said

The investigation has centered on the fact that guards appeared to make no effort to open cell doors for the detained men — almost all from Guatemala, Honduras, Venezuela and El Salvador — before smoke filled the room in a matter of seconds.

The deaths caused frustration, and may have played a role in a mass march late Wednesday afternoon by hundreds of migrants, who began walking toward a U.S. border crossing in the belief that American authorities would let them through.

Adding to anger over the deaths was pent-up frustration of migrants who have spent weeks trying to make appointments on a U.S. cellphone app to file asylum claims. Rumors spread among the migrants that they might be let in into the U.S.

Jorman Colón, a 30-year-old Venezuelan migrant, walked hand-in-hand with his 9-year-old daughter, saying he had heard on social media that acquaintances had gotten through.

“We want to turn ourselves in,” Colón said, referring to the first step in the asylum process.

Several hundred of the migrants crossed the shallow Rio Grande from Mexico toward the U.S. and approached a gate in the border fence that separates El Paso and Ciudad Juárez. Armed agents stood guard at the U.S. gate entrance.

Venezuelan migrant Victoria Molina, 24, complained that “the app never gives us an (appointment) date.”

A group of about 50 migrants initially approached a Border Patrol vehicle and personnel and sat or kneeled on the ground. About 25 of them were then led in single file through the gate into the U.S. and onto a white school-bus style vehicle that drove away.

U.S. officials said Wednesday night that a total of about 1,000 migrants had crossed the river and were being processed in an orderly manner. It was unclear if they would be allowed to remain or be bussed to a formal border crossing for expulsion.

Smoke began billowing out of the migrant detention center late Monday after a group of detained migrants set fire to foam mattresses, to protest what they thought were plans to move or deport them.

Immigration authorities said they released 15 women when the fire broke out, but have not explained why no men were let out.

President Andrés Manuel López Obrador said Wednesday that both immigration agents and security guards from a private contractor were present at the facility.

Also Wednesday, Pope Francis offered prayers at the end of his general audience for those who died in the “tragic fire.”

Leaked surveillance video shows migrants, reportedly fearing they were about to be moved, placing foam mattresses against the bars of their detention cell and setting them on fire.

In the video, later confirmed by the government, two people dressed as guards rush into the camera frame, and at least one migrant appears by the metal gate on the other side. But the guards don’t appear to make any effort to open the cell doors and instead hurry away as billowing clouds of smoke fill the structure within seconds.

It was unclear if the two guards actually had the keys, but authorities suggested Wednesday that they should have gotten them or broken the lock — a highly difficult task, given the quick spread of smoke.

U.S. authorities have offered to help treat some of the nearly 30 people who are hospitalized in critical or serious condition, most apparently from smoke inhalation.

The migrants were stuck in Ciudad Jaurez because U.S. immigration policies don’t allow them to cross the border to file asylum claims. But they were rounded up because Ciudad Juarez residents were tired of migrants blocking border crossings or asking for money.

“There were several complaints from neighbors about a group of migrants, we don’t know if it was this group or another, that was allegedly acting aggressively, asking people in the street for money, demanding it,” said Rodríguez.

The high level of frustration in Ciudad Juarez was already evident earlier this month when hundreds of mostly Venezuelan migrants tried to force their way across one of the international bridges to El Paso, acting on false rumors that the United States would allow them to enter the country. U.S. authorities blocked their attempts.

After that, Ciudad Juarez Mayor Cruz Pérez Cuellar started campaigning to inform migrants there was room in shelters and no need to beg in the streets. He urged residents not to give money to them, and said authorities removed migrants intersections where it was dangerous to beg and residents saw the activity as a nuisance.

On Wednesday, the mayor told AP his office had not received any report of rights abuses of migrants in detention facilities. He insisted that his government shared no responsibility for what happened.

“It’s a terrible tragedy that pains all of us. We are grieving,” he said, adding that authorities should “come down with the full weight of the law on those responsible — the people that for instance, didn’t open the doors for the migrants.”

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Associated Press videojournalist Alicia Fernández and writers Maria Verza Guadalupe Peñuelas in Ciudad Juarez, Mark Stevenson in Mexico City, Sonia Pérez D. in Guatemala City and Elliot Spagat in San Diego contributed to this report.