Slovak Prime Minister Robert Fico has offered a one million euro ($1.2m) reward for anyone who comes forward with information about the murder of an investigative journalist.

Jan Kuciak, 27, and his partner Martina Kusnirova were shot dead in their home.

Both were found with single gunshot wounds on Sunday.

Several newspapers in Slovakia have printed Kuciak's last article, which links the Italian mafia to high-level political corruption.

Mr Fico held a press conference on Tuesday, where he stood by piles of banknotes that the government is promising in exchange for information about the killings.

• Slovakia shocked by journalist's shooting

Press speculation about the motive for the mysterious murders has been rife in Slovakia.

Kuciak's colleagues in the media say that authorities should look no further than his latest piece, published by Slovak outlets overnight.

In the unfinished article, he alleges that businessmen in eastern Slovakia - with links to Calabria's notorious 'Ndrangheta mafia - are embezzling EU structural funds.

He also claims that they have political ties in the country.

Both Kuciak and his girlfriend were discovered with single fatal gunshot wounds, police say

According to earlier media reports, Maria Toroskova, a senior advisor to PM Fico, was among those being probed.

"Do not link innocent people without any evidence to a double homicide," Mr Fico told journalists on Tuesday.

"It's crossing the line. It's no longer funny."

During the Tuesday press conference Mr Fico complained about "political abuse of a tragedy" after opposition politicians held a news conference making accusations linking the ruling Smer party to the killings.

Opposition groups have called for fresh anti-corruption protests in the capital Bratislava on Wednesday.

Police President Tibor Gaspar said authorities had questioned 20 people since Monday, and had contacted the Czech Republic and Italy about the investigation.

Mr Gaspar also confirmed the EU's police agency Europol had offered their specialist assistance with the investigation.

The chief has said the motive was "most likely" related to Kuciak's investigative journalism, and has warned reporters about publishing details of the case.

"How can we do our work effectively if you are alerting some people who may be involved?" he said.

The killing of the couple has triggered calls for fresh anti-corruption demonstrations

Like other ex-communist countries, Slovakia enacted far-reaching law and justice reforms in order to qualify for EU membership.

Kuciak had been working for Aktuality.sk, an online unit of Swiss and German-owned publisher Ringier Axel Springer, for three years.

He, like fellow murdered Maltese journalist Daphne Caruana Galizia, worked on the Panama Papers scandal.

The Slovak Prime Minister has said he would meet media outlets to assure them "that the protection of freedom of speech and the safety of journalists is our common priority and that it is extremely important to my government".