Three possibilities here, as I see it. First, the witness wants to give that information to distract from the real perpetrators. Why? Is he involved?
Second the witness could do this for personal attention or glory.
Third, the Police are lying. The description is vague but accurate as far as it goes. The witness is involved. The perps don't intend to get caught and let everyone know for sure it was a jihad, but want that possibility to simmer in our minds.
Two of the three possibilities have our witness as involved, which screws up the plan, UNLESS the perps want to be caught.