BUDAPEST, Hungary >> With thousands of migrants pouring out of Afghanistan and the Middle East, the business of smuggling them across the Balkans into the European Union has grown even larger than the illicit trade in drugs and weapons, law enforcement officials said.

In Greece alone, there are 200 such smuggling rings, said Col. Gerald Tatzgern, head of the Austrian police service fighting human trafficking.

“It has developed into a business worth billions,” said Johanna Mikl-Leitner, the interior minister of Austria. Smugglers have spread out through the region, in Bulgaria, Hungary, Macedonia, Romania and Serbia, she added.

Their presence is evident at Budapest’s Keleti train station, where thousands have had their route to the West cut off by Hungarian officials. The smugglers’ agents roam the crowd, quietly offering rides to Austria for hundreds of dollars.

But the memory is fresh of the 71 migrants found suffocated in the back of a sealed truck along an Austrian highway last week, abandoned by their smugglers.

In another case, the police rescued 24 Afghans from a sealed truck after a chase through the streets of Vienna. “One hour longer and they would have been dead,” Mikl-Leitner said.

Rob Wainwright, head of Europol, Europe’s police agency, told Newstalk, an Irish radio network, that an estimated 30,000 people were involved in human smuggling gangs. Europol has opened 1,400 cases this year involving human smuggling, he said.

Officials continue to arrest suspects involved in the operation that led to the 71 deaths.

While members of smuggling groups that use a sea route into Italy have been prosecuted, this case is the first against a land-based ring, and may be the first in which the police have linked both a driver and his accomplices to deaths, said Melissa Fleming, chief spokeswoman for the United Nations refugee agency.

Until now, Fleming said, smugglers were “able to commit mass murder with impunity.”

Prosecutors in Bulgaria released the names and some details of the five citizens arrested. Four are in custody in Hungary; the fifth awaits extradition from Bulgaria. An Afghan national — who, like the others, has a long criminal history, officials said — was also arrested in the case.

Some of the Bulgarian suspects lived elsewhere in Europe recently, said Rumiana Arnaudova, chief spokeswoman for the Bulgarian prosecutor’s office.

One of the men, Metodi Georgiev, is from Lom, Bulgaria, on the Romanian border. Several residents who declined to give their names for fear of antagonizing the smugglers said Georgiev had been living in Austria and Germany. They said he had not been one of the truck’s drivers, but a lookout and recruiter.

Another suspect, identified as Tsvetan Tsvetanov, was arrested on a European warrant issued by the Austrian police. He appeared Thursday in Montana, Bulgaria.

Ushered into the courtroom with his hands and feet shackled, Tsvetanov denied being one of the truck’s drivers.

He told the judge he was unemployed and had not been traveling abroad. But outside the court, some of his relatives said he had recently returned from Germany, where he had gone in search of a job.

A third suspect, Vetsislav Todorov, is from a village near Vratsa, Bulgaria, local news media reported. His brother was quoted by Btv, a Bulgarian network, as saying the leader of the smuggling ring was from Lom.

Prosecutors identified the two other suspects as Kasem Saleh and Ivailo Stoyanov, but provided no details about their residences.

The Afghan suspect, Samsooryamal Lahoo, also in custody in Hungary, was previously picked up in 2013 after traveling to Germany without a passport, said Frank Passade, a spokesman for state prosecutors in Bremen.

Officials are expected to give more details Friday on the time and cause of the 71 deaths. But the deteriorated state of the bodies and the lack of identity documents may mean that some of them — 59 men, eight women and four children — will never be identified, a police spokesman, Helmut Marban, said Thursday.

Officials said the smuggling groups in the Balkans varied in size and sophistication. Usually, they are local organized crime groups that have simply seized on a moneymaking opportunity, said Izabella Cooper, spokeswoman for Frontex, which monitors the European Union’s borders. They normally hire Afghan or Syrian representatives to act as their agents on the ground, handling contact with potential customers, Cooper said.

“If a migrant has a lot of money, smugglers can get them a forged passport or a stolen ID card with a visa, together with a plane ticket to a chosen European country,” she said. “But this option is affordable only for a handful of people.”

Migrants can sometimes plot a route from Turkey to Germany with one central coordinator and are passed from gang to gang, paying as they go, said Livia Styp-Rekowska, senior immigration and border management specialist at the International Organization for Migration.

“The one phenomenon that we definitely see in this region is the growth of the smuggling operation online,” Styp-Rekowska said. “There are all sorts of online advertisements targeting Syrians, written in Arabic.”

The migrants have become wary of dealing with the smugglers, both for fear of injury or death and because there are many stories of migrants’ being cheated by phony smugglers.

One group, preying on the migrants huddled outside the Keleti station this week, promised rides to the Austrian border, but instead placed the migrants in the back of a windowless van and drove several times around Budapest’s outer ring road, depositing them by a suburban shopping center with entry gates that look vaguely like a border crossing.

Others are abandoned in the woods and threatened if they complain. Still others are robbed and left on remote roadsides.

“We explain the options, but the smugglers are so tempting,” said Zoltan Bolek, head of the Hungarian Islamic Community, a service group. “Taxis willing to take them to the Austrian border are everywhere. It costs 1,000 euro, and if they are lucky, they actually get there.”