Until now, Ms. 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 Mr. 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, Mr. 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.