A terror attack is the least likely version to explain the incident with a suspected "homemade bomb" found in a bus traveling from Prague (Czech Rep.) to Varna in Bulgaria, a counter-terrorism expert has said.

Most people traveling on the bus were Bulgarians. The "device" (as described by drivers and passengers alike) was reportedly found in a luggage that belonged to a passenger who got off in Brno, in the Czech Republic, and did not return, leaving his personal belongings.

All passengers are now safe and in Bulgaria.

Though neither Bulgarian nor Hungarian police have provided much information about the incident which took place in Budapest on Sunday afternoon, a number of Bulgarian experts are already looking for an explanation.

"I hear the driver say several times... that it was a time bomb. I am curious, how many times has this person seen a time bomb," Slavcho Velkov, who works for a number of security think-tanks, argued in an interview with the Bulgarian National Television on Tuesday.

He added that imitating "a time bomb" using non-explosive materials just to scare off anybody was an easy task, showing how a "mock bomb" could be made out of everyday objects.

In his words, the "attack" could have been carried out by a competitor to the transport company operating the bus.

Velkov added a "personal vengeance" shouldn't be ruled out, though he didn't elaborate.

Omega Group, the company concerned, suffered another similar incident last year, when another one of its buses was set ablaze.