Its just a standard trick to slime conservatives. They know "right wingers" are not responsible, but if the police haven't come out and said who is responsible, then that gives the press time to speculate without being liars. They get a free 24 hours or so to brainwash their viewers into thinking right wingers are lurking around every corner, ready to blow them up. When the truth comes out and it isn't evil right wingers, they stop and refocus the audience onto something else. But they have succeeded because they were able to program their audience into thinking that conservatives are violent.
THey do it it with almost every terror attack until the facts are known.