The bodies of 27-year-old Jan Kuciak and his partner were found on Sunday evening in their house in the town of Velka Maca, east of the capital, Bratislava, police president Tibor Gaspar said.
Police went to the house at the request of a worried family member.
Gaspar said the killings “likely have something to do with his investigative activities.” He declined to elaborate.
Slovak Prime Minister Robert Fico said if that if that were the case, it would be “an unprecedented attack on freedom of the press and democracy in Slovakia.”
“We have to find those who did it as soon as possible and ensure the safety of all journalists,” Kiska said in a statement.
Gaspar said the reporter was shot in the chest while the woman was shot in the head. He said they were estimated to have been killed between Thursday and Sunday.
He added that Slovakia “has never faced such an unprecedented attack on a journalist.”
Kuciak was working for Aktuality.sk news website. He focused mainly on tax evasion.
“We are shocked and stunned by the news that Jan Kuciak and his partner were apparently victims of a cruel attack,” publisher Ringier Axel Springer Slovakia, to which Aktuality.sk belongs, said in a statement.
“We mourn with the family, the friends and the colleagues; we will do everything to support the investigating authorities to bring the perpetrator to justice.”
Kuciak said last year he filed a complaint with police after the businessman, Marian Kocner, threatened him following another story about him. Kuciak then complained that police failed to act.
Gaspar said all people who were in touch with the journalist will be investigated.
He said police will provide protection for an unspecified number of other reporters from Aktuality.sk. He gave no details.