I just spoke to Michael C. Ormsby, the U.S. Attorney for the Eastern District of Washington, who tells me that the formal complaint against Colville resident and backpack bomb suspect Kevin William Harpham, 36, will remain sealed for now.

"We requested it be sealed because we don’t want to jeopardize our continuing investigation,” Ormsby said.

Does that mean there are more suspects out there in the backpack bomb case?

Not that Ormbsy is aware of.

But, he added: "We want to continue this investigation to make sure that if there are others involved we can continue to identify them.”

Hence the effort to not tip them off, if they are out there, by unsealing the complaint—and the evidence against Harpham—right now.

And what about the explosion that was reportedly heard before, or during, the FBI's arrest of Harpham today?

Ormsby told me that the FBI had been prepared to make an arrest today—suggesting the explosion didn't lead to the arrest—and he speculated that the explosion might have been a diversionary tactic employed by FBI agents closing in on Harpham, who is set to appear in court in Spokane later this afternoon to enter a plea.