it is considered the biggest test yet for the iraqi army. iraq's prime minister ordered the start of a major offensive to take back the city of tikrit from isis. thousands of iraqi and militia forces are gathering 70 kilometer from the city which has been under isis control for eight months. in moscow thousands marched in mourning of the killing of boris nemtsov. the outspoken critic was gunned down on a bridge near the kremlin. more than 48 hours later, his killer or killers remain at large. investigators are pursuing multiple leads in nemtsov's case including the possible that the opposition may have been behind his death. police have released a vague image of what his killer might look like. our senior international correspondent, fred pleitgen reports. >> reporter: the police appear to have gone away from the theory that them son of walking