He was nearly killed in a car bombing while exiting a hotel. The car bomb turned out later to be a completely unrelated event. (Grissom Versus the Volcano)
Behind the scenes
The TV guide summary of Grissom Versus the Volcano states that Atwater suspects terrorists, but the episode itself does not show that. However, Atwater asking Grissom if there was distinct patterns on the pipe bomb could be his suspicion of terrorists.