We wanted to verify that this number really was Luka’s.
So a few days later, with us present, the man agreed to put the person he said was Hussam Luka on speakerphone, and ask him questions which we believed only Luka would know the answer to, based on information we had gathered during our investigation.
The voice on the other end of the phone sounded confused, but answered the questions instantly and correctly.
He refused, however, to speak to us directly.
But our contact later agreed to call Luka again and allow us to try to interview him.
The same voice answered, and we introduced ourselves, explaining that we had questions about serious allegations relating to massacres and torture committed by the former regime official.
“I can’t take any risk talking on the phone in the position I’m in,” Luka said, his tone anxious.
Asked if Russia was blocking him from talking, Luka said: “I cannot talk. I can’t explain. You have to understand.”
When asked if he would meet us outside Russia, he said: “It’s difficult.”
He then hung up the phone.
We later obtained a second number which we were told was also for Luka – this time registered in Syria and linked to social media and messaging apps which he appeared to be still using.
Over a period of a day, the BBC was able to locate him in two suburban areas on the outskirts of the Russian capital, Moscow.
The locations seemed far removed from the luxurious lifestyle Assad is reported to be leading in exile in Russia, and to the life Luka himself once led in Syria.
We knew we could not approach him in person in Russia, where Assad’s henchmen are shielded from international journalists and police, so we decided to try putting our allegations to him over the phone once again.
We rang the number and the same voice as before answered, the line crackly.
He asked who was calling, and when told it was the BBC, he immediately hung up.
We sent a message over WhatsApp, asking for his response to the many allegations against him, but he did not reply.
Confirmation that Luka is in Moscow is difficult news for those in Syria seeking justice. Russia is highly unlikely to extradite officials from the regime that it long supported.
The Russian government did not respond to BBC requests for comment about Luka’s presence there, or whether it would ever consider an extradition request.
Back in Homs, Mohammed says he will never let go of the hope that Luka, and other senior members of the Assad regime, will one day face justice.