Slovak police on Thursday raided houses linked to alleged members of the Italian mafia in connection with the slaying of an investigative journalist and his girlfriend.

Slovakia's national police chief Tibor Gaspar told reporters in the eastern city of Kosice that seven men were detained in the raids in the towns of Michalovce and Trebisov.

Gaspar says they were detained as "suspects" with approval from the state prosecution. The men ranged from 26 to 62 years old. Only their first names were given.

The bodies of 27-year-old Jan Kuciak and his girlfriend, Martina Kusnirova, were found Sunday evening in their house in Velka Maca, east of the capital, Bratislava. They had both been shot.

Kuciak's last, unfinished story was about the activities of the Italian 'Ndrangheta organized crime syndicate in Slovakia.

Slovak media reported that one of those detained Thursday was Antonino Vadala, an Italian who did business with at least two officials close to Slovakian Prime Minister Robert Fico. Those officials — a senior adviser to Fico and the secretary of the country's security council — resigned from their government posts Wednesday.

Experts from the FBI and Britain's Scotland Yard have now joined other foreign agencies — including the Czech and Italian police and Europol — who are helping Slovak authorities investigate the killings, Interior Minister Robert Kalinak said.

In a potential problem for Fico's coalition government, the chairman of a junior partner demanded Thursday that Kalinak resign. Bela Bugar of the Most-Hid party of mostly ethnic Hungarians says that would increase public trust in the investigation.

The opposition has already demanded that Kalinak and police chief Gaspar resign. Thousands of people rallied last year in Bratislava and other cities to demand their resignations, saying they blocked the proper investigation of corruption scandals.

Thursday's requests will likely cause tensions in the governing coalition, as Kalinak is a key ally of Fico in his leftist Smer-Social Democracy party.