The murder of a Slovakian journalist may be linked to organised crime, according to reports.
Police were sent to the couple’s home after a concerned family member raised the alarm.
Mr Kuciak, a journalist for news website Aktuality.sk, is said to have been working on a story about possible Italian mafia involvement in fraud linked to European Union subsidies in Slovakia when he was killed.
In an interview with Slovakian daily newspaper Sme, Slovak-based Canadian journalist Tom Nicholson said Mr Kuciak had told him shortly before his death that he was working on a story about possible Italian mafia activity in the east of the country.
Mr Nicholson said he was ready to testify, but has not been approached by police yet.
“I'm not sure what caused Jan's death but I bet my life that this is so,” he said.
Police said the killings were probably linked to Mr Kuciak's reporting, which also covered tax evasion.
His last story, published on 9 February, focused on suspected tax evasion connected to a Bratislava apartment complex with links to the Slovak government.
In a statement, Slovakia President Andrej Kiska said the perpetrators needed to be found “as soon as possible” to “ensure the safety of all journalists”.
Prime Minister Robert Fico said: “If it is proven that the death of the investigative reporter was connected with his journalistic work, it would be an unprecedented attack on freedom of speech and democracy in Slovakia.”
Mr Kuciak is the first journalist to be killed in Slovakia and the second reporter to have been murdered in a European Union country in the past five months.
Last October, Maltese journalist Daphne Caruana Galizia was killed in an apparently targeted car bomb attack after shining a spotlight on alleged corruption in the country.
Publisher Ringier Axel Springer Slovakia, the owners of the website Aktuality.sk, said it was supporting the investigation into the murder of Mr Kuciak and Ms Kusnirova.
“We are shocked and stunned by the news that Jan Kuciak and his partner were apparently victims of a cruel attack,” the publisher 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.”
The case has been condemned by the NGO Reporters Without Borders (RSF).
“An investigative reporter has yet again been murdered in a European Union country,” said Pauline Ades-Mevel, the head of RSF’s EU-Balkans desk.
“We call for an investigation in order to establish the exact circumstances of Jan Kuciak’s death and we demand that the authorities shed all possible light on this case, especially as he and those close to him had been threatened in recent months.”