BUDAPEST, Hungary (AP) — Hungarian police have detained a fifth man suspected of being involved in the deaths of 71 migrants found in a truck in neighboring Austria.

The Bulgarian national was arrested on Saturday evening, a national police statement said Sunday. Police said they will seek to have him held in custody on suspicion of human trafficking, but gave no further details about him.

On Saturday, a court in the central Hungarian city of Kecskemet, where prosecutors say the truck departed, placed four other suspects under preliminary arrest pending possible indictment in the case.

The three Bulgarians and an Afghan were arrested Thursday in southern Hungary, after the truck with the dead migrants was found earlier that day parked along the Budapest-to-Vienna highway.

Austrian experts are performing autopsies on the migrant victims — 59 men, eight women and four children.

Austrian police have said the migrants likely suffocated to death. As of Sunday, 16 autopsies had been performed but there was no conclusive information yet on the cause of death, police spokeswoman Alexandra Hareter said. The process is expected to continue for several days.

The identity of the migrants remains unclear. Police have set up a hotline for people who may have information on who was aboard the truck and also are hoping for clues from examining an unspecified number of cellphones found in the vehicle.

Investigators also found a Syrian passport in the truck.

"One can suspect that this was a Syrian group, or (that there were) a few Syrians," Burgenland province police chief Hans Peter Doskozil told the Austria Press Agency. "But it could be mixed. We don't know at this point."