Federal authorities found more than 600 Central American migrants hidden in the refrigerated containers of three tractor-trailers traveling in Tamaulipas on Thursday night.

Tamaulipas authorities said that the army and National Guard stopped the trucks at a military checkpoint on the Ciudad Victoria-Monterrey highway in the municipality of Hidalgo. A total of 652 migrants, including 197 unaccompanied minors, were found in the containers. Another 158 minors were traveling with adults.

Authorities said the group was made up of 564 Guatemalans, 39 Hondurans, 28 Nicaraguans, 20 Salvadorans and one person from Belize. Four people were arrested in connection with smuggling the migrants in the trucks, which authorities said had departed from Puebla and were headed for Monterrey.

The migrants were taken to state police facilities in Ciudad Victoria on Thursday night and some received medical assistance from the Red Cross. Some were taken to federal Attorney General’s Office facilities on Friday morning, the newspaper Reforma reported.

Tamaulipas health authorities said that nine of the migrants had tested positive for COVID-19, but most had only mild symptoms. State Health Minister Gloria Molina Gamboa said they had been placed in isolation and were under medical supervision.

Other migrants had ailments including dehydration and respiratory problems. At least two pregnant women were among those traveling in the refrigerated containers.

The discovery comes amid a record influx of migrants into Mexico. Some 147,000 migrants were counted in Mexico in the first eight months of the year, and almost 80,000, including large numbers of Haitians, applied for asylum here.

Many have entered the country via Mexico’s border with Guatemala in Chiapas before making their way to Tapachula, where there are currently more than 100,000 migrants, according to civil society organizations. Most migrants who have arrived this year are Central Americans, Cubans and Haitians. However, almost 1,700 Africans from 35 countries have also sought asylum in Mexico this year after crossing into Chiapas.

Among them are 329 people from Senegal, 247 from Ghana, 179 from the Democratic Republic of the Congo, 151 from Angola and 111 from Guinea.

With reports from El Universal, Reforma and El Orbe