BUDAPEST, Hungary -- A fifth man suspected of being involved in the deaths of 71 migrants found in a truck in neighbouring Austria has been detained, Hungarian police said Sunday.

The Bulgarian national was arrested on Saturday evening, a police statement said. 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.

Three other 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."

Hungarian police have detained 22 people suspected of human trafficking in several cases over the weekend. They included nine Hungarians, seven Romanians, three Serbians, as well citizens of Austria, Lithuania and Poland.

Among the migrants found in the vehicles were 160 Syrians, 21 Iraqis, 20 Afghanis and two from Pakistan.

Romania's foreign ministry said Sunday that 26 Romanians had been detained in Hungary recently, suspected of human trafficking.

Police in Austria's Burgenland province, where the truck with the dead migrants was found last week, said Sunday that they arrested five suspected smugglers in four separate incidents in the past two nights. The suspects -- three Hungarians, a Croat and an Italian -- had brought a total of 36 people into Austria.

The Austrian government said that it was stepping up checks in areas near the country's eastern border starting Sunday evening, and officials will stop and check large vehicles in which migrants could be hidden. The measures will remain in place until further notice.

At Budapest's Keleti train terminal, meanwhile, several hundred migrants protested at the terminal's main gate, demanding that Hungarian authorities allow them to travel to Germany.

Migrants applying for asylum after crossing over from the southern border with Serbia are registered by authorities and usually sent by train to refugee centres, passing through Budapest. They are supposed to stay in Hungary until their asylum requests are settled but many, along with many other migrants who have avoided being registered, quickly try to go to richer EU countries, especially Germany.

"We have tickets but we are told not to go," said Alim, a 29-year-old from Syria who said he had friends in Frankfurt and that he didn't want to give his surname because he doesn't yet have documents to legally reside in the EU. "We want to go now!"

Geir Moulson in Berlin and Alison Mutler in Bucharest, Romania, contributed to this report.