Ching Chiang-class boat briefly stuck on sandbar near Zuoying Naval Base

Navy operates 6 Kang Ding-class frigates from Kaohsiung base

Group of 6 were conducting repairs at sea off Zuoying Navy Base

One crew member in critical condition after helicopter caught fire

Officer spent leave in Keelung, reported ill before entering base

Prosecutors say Chen did not contact foreign agents

Investigation has not yet found motive for suspects entry into Zuoying Navy Base