He makes no claim that the snipers came from outside, but he does hint that the evidence points back to either a faction in the Maiden protests, or a faction paid for by the government, which he seems to have come to the conclusion through their refusal to investigate the crime.
He says "all the evidence shows" I'm assuming this is the evidence so far gathered by British forensic investigators, who were sent to the Ukraine last week to investigate the crime scene. Even though he is technically passing on a "claim/rumour" as it's not been proven in a court of law, he is still talking in the context of evidence discovered through the forensic investigation, which provide evidence to prove that the bullets came from the same gun and I'm assuming don't match police issued weapons.