"APB: At Least Two Missing "Law Enforcement Personnel," Last Seen at Ibragim Todashev Homicide Scene". The 'law enforcement personnel' written out of the story are, of course, the assassination team.
To summarize, the FBI was disturbed by the first interview, as Todashev indicated he knew the brothers were set up. It was thus imperative to:
1. reinterview Todashev to find out what he knew, and more importantly, identify whoever else knew; and
2. liquidate Todashev.
Why didn't they just arrange for a drive-by shooting, or a home invasion/murder? They needed to interview him to find out who else they had to murder, and then immediately liquidate Todashev before he could talk to anyone else. The incompetence of the liquidation was probably caused by Todashev realizing what was going to happen, fighting for his life rather effectively (as he was a trained fighter), resulting in the panicked murder, and a most ludicrous attempt at a cover-up.