Interfax news agency reported that a probe was launched in 2017 into why Oleg Smolenkov was allowed to leave for Montenegro on vacation.
"With this kind of president, with his knowledge of foreign policy, who knows what is going on? Foreign policy is not his big thing."
The Republican senator said Trump is "playing right into Putin's hand."
The Balkans could be the first test of whether President Donald Trump will side with NATO or Russia.
When Tucker Carlson asked why his son should be sent to defend Montenegro, Trump said: "I understand what you're saying. I've asked the same question. Montenegro, you know, is a tiny country with very strong people."
It was the first time Russia attempted to use violence outside of the former Soviet Union to achieve its political aims, a report said.
Italian police uncovered the fraud after the Montenegrin foreign ministry refused to pay for the man's bill at a luxury resort.
Moscow said it reserved the right to respond to Montenegro's "hostile course," while Washington praised the country for "choosing its own alliances."
Russia's condemnation follows President Donald Trump's statement that U.S.-Russia relations have hit a new low.
Moscow's attempts to regain influence in the Balkans shows the perils of backing off Russia.
Tillerson argued that Montenegro's membership in the alliance would support greater integration, democratic reform, trade, security and stability among its neighbors.
Two opposition alliances said the re-elected government created an atmosphere of fear.
According to three organizations, no real evidence of Martinović's involvement with criminal activity has been presented.
Andy Murray is expected to take part but Novak Djokovic's participation remains unknown.
It is an important statement to Putin that we will not be intimidated by bullying.
Kremlin spokesman Dmitry Peskov said that Moscow would move to protect its interests
NATO chief Jens Stoltenberg said: 'This is the beginning of a very beautiful alliance.'
Fears spread in eastern Europe as one person is quarantined in Montenegro and 14 under supervision in Serbia