At least 39 people died in a fire at a migration center in Ciudad Juarez, a city on Mexico’s border with the United States, officials said Tuesday.

Authorities said the fire at the office of National Migration Institute (INM) broke out after they picked up a group of migrants from the streets of the city, and detained them.

The cause of the fire or the victims’ nationalities have not been released by Mexico’s National Migration Institute, who have launched an investigation into the blaze, which left 29 people injured.

Mexico’s President Andrés Manuel López Obrador did provide some details.

“What we know so far is that migrants from Central America and some from Venezuela were in that shelter. We still do not know exactly the names and nationalities of those who unfortunately lost their lives,” Lopez Obrador said.

“This had to do with a protest that they started after, we assume, they found out that they were going to be deported, and as a protest, they put mattresses from the shelter at the door of the shelter, and they set fire to them and they did not imagine that this was going to cause this terrible accident,” he said.

“It is very sad that this is happening,” added López Obrador.

And Andrea Chavez, Ciudad Juarez’s federal deputy, tweeted of her “deep sadness” on Tuesday.

“It is with deep sadness and grief that we learned of the fire that occurred inside the INM in Ciudad Juárez,” she said.

We will wait for the official information and, from this moment on, we send our condolences to the families of the migrants. FGR initiated the investigation,” Chavez said.

Body bags were lined up near the scene of the fire, which had been extinguished, Reuters reported a witness as saying. Most of the migrants at the center were Venezuelan, the witnesses added.

“I was here since one in the afternoon waiting for the father of my children, and when 10 p.m. rolled around smoke started coming out from everywhere,” 31-year-old Viangly Infante, a Venezuelan national, told the agency.

Her husband, 27-year-old Eduard Caraballo, was inside the detention center and survived by spraying water on himself, according to Infante, who said she saw many dead bodies.

The blaze is one of the worst in recent years in Mexico, which has seen record levels of crossings at its border with the US.

Earlier this year, the Biden administration ramped up efforts to curb the number of migrants crossings at the border.

In February, it released a new rule that largely prohibits migrants who traveled through other countries on their way to the shared frontier from applying for asylum in the US, marking a departure from a decadeslong precedent in proposed regulations reminiscent of Trump-era policy.

CNN has reached out to Mexico’s migration authorities for statement on the fire.