In February, Russian investigative journalist Maxim Borodin published a series of bombshell reports about the secret, substantial presence of Russian mercenary forces in Syria. On Sunday, he died, following a mysterious fall from his fifth-floor balcony.

Now, a journalists’ advocacy group is calling for an investigation into his “suspicious” death — even though his own editor-in-chief has said there’s not yet any hard evidence of foul play.

Local police said they’re investigating “several versions” of the death of Borodin, 32, who worked for an outlet called Novy Den in the city of Yekaterinburg. The cops said in a statement they “haven’t ruled out that it was an accident.”

“All hypotheses should be considered, including the possibility that he was murdered in connection with his investigative reporting,” Reporters Without Borders said in a press release Monday.

Borodin had helped shed light on Russia’s “shadow army,” the thousands of Russian military contractors secretly fighting on the side of President Bashar Assad in Syria.

The Kremlin has tried to downplay reports that some 300 Russian mercenaries were killed or injured in a firefight with American forces in Syria on February 7. But days after that clash, Borodin wrote about the deaths of men thought to have tangled with U.S. troops as mercenaries.

Polina Rumyantseva, Borodin’s editor-in-chief, told VICE News that so far no hard evidence has emerged to indicate either foul play or suicide. “We cannot, as of now, connect this tragic event with his professional activities,” she said by phone from Yekaterinburg. “It looks like he stepped out for a smoke, and then something happened.”

But a friend of Borodin’s recounted a troubling incident that took place the day before Borodin’s death.

Vyacheslav Bashkov wrote in a post on Facebook that he received an anxious 5 a.m. phone call from Borodin on Wednesday.

Borodin told Bashkov that his apartment was surrounded by security forces in camouflage and masks.

“Max sounded distressed, but not hysterical or drunk,” Bashkov wrote.

Borodin told his friend that he thought his apartment would soon be broken into and searched. He asked Bashkov to make some calls and help him track down a good media lawyer. Borodin said he suspected the security forces might be waiting for a court’s permission to enter his apartment.

But a short time later, Borodin called his friend back to reassure him everything was alright.

“Borodin called again, sounding guilty, and said he was mistaken, that it was apparently some sort of exercise,” Bashkov wrote. “He apologized. Maxim investigated crimes as a journalist, so paranoia was understandable.”

On Friday, Bashkov saw on the news that his friend had been found below his balcony. Borodin died in the hospital on Sunday without regaining consciousness, Novy Den announced.

Reporters Without Borders tweeted that the incident was “suspicious,” and called for a “thorough, impartial investigation.”

Harlem Désir, representative on Freedom of the Media for the Organization for Security and Cooperation in Europe, tweeted that Borodin’s death “is of serious concern,” and likewise called for a thorough probe.

“We must find out for sure that no one helped him to fall off the balcony,” Bashkov wrote on Facebook.

The local police declined to comment further to VICE News.

Borodin’s editor, Rumyantseva, said that so far the available evidence suggests the most likely explanation is that Borodin’s death was an accident.

“As journalists, if we had even a shadow of a doubt that what happened wasn’t just a coincidence, we’d ring bells from the rooftops,” she said. “So far, we don’t have that.”

Borodin had been attacked once before in relation to his work. He was clubbed over the head with a metal pipe by unknown assailants after giving an interview about a controversial Russian film, he said in October. That movie, Matilda, portrays a love affair by the last Russian tsar, Nikolas II, who was canonized by the Russian Orthodox Church in 2000. Russian religious hard-liners have raged against the film as blasphemous.

According to the Committee to Protect Journalists, 58 reporters were killed in Russia between 1992 and 2017. Of those, 38 were murdered for reasons directly related to their work, and 33 were murdered with impunity.