applies the the ukrainians were responsible. a claim that ukrainian authorities have denied one senior official saying we are not a criminal state. fred pleitgen is in moscow. where do things stand on this investigation? reporter: a lot of this is still very much a mystery. at such dazzling time in russia as you have russia s special military operation, as they call it, in ukraine, going on. so there were, of course, someone in the pretty high echelons in russia who believe that maybe the ukrainians might be behind all this. but right now, it still remains absolutely unclear. the latest from the investigation is that they say the explosive yield of that charge was the equivalent of about 400 grams of tnt. they say the bomb was placed inside the car. they ve recovered some parts and sent it in for a forensic investigation as well. still very much unclear who might be behind all this. here s what we re learning. reporter: a car engulfed in a massive fire ball on a highway o