Bulgaria’s interior ministry and local police say two police officers were killed Thursday when a bus carrying at least 47 migrants that had entered the country illegally rammed their vehicle as they tried to stop it.

Police say the incident occurred about 5 a.m. local time in Burgas, a port city on the Black Sea. Burgas Police Chief Kaloyan Kaloyanov told reporters at the scene that police had tried to pull over the bus after it drove through two border checkpoints.

The chief said the bus had entered a residential area when police officers stopped their car in its path to block it. The bus allegedly rammed the police vehicle and drove over the top of it before smashing into a bus stop. Two officers inside the car were killed instantly.

No other injuries were reported and an investigation into the case has been launched.

Police said the migrants on the bus were from Syria.

It is unclear what charges may be filed, but local prosecutor Georgi Chinev called the ramming of the police car a “conscious, purposeful, intentional act.”

The news website BalkanInsight reports five people were arrested at the scene on human trafficking charges.

On Twitter, BNN Bulgaria — an affiliate of Bloomberg news — reports citing local police that the bus was a deregistered vehicle that had been sold for scrap with phony license plates.

Bulgarian media reports there was a similar incident Sunday in the western Bulgarian town of Godech. In that case, a bus carrying illegal migrants crashed into a tree, killing the driver.

Bulgaria, a Balkan country of 7 million people, is located on a major route for migrants from the Middle East and Afghanistan to Europe.

Some information for this report came from the Associated Press.