KUALA LUMPUR, Malaysia — Pastor Raymond Koh was driving along a tree-lined suburban street near Malaysia’s capital 15 months ago when masked men in black SUVs forced his sedan to a halt. Within 45 seconds, they had commandeered his car and taken him away. He has not been seen since.

Family members believe that government operatives were behind the military-style abduction and that, since then, the police have helped cover it up. Mr. Koh, they say, was most likely targeted in the belief that he preached Christianity to Muslims.

Now, with the ouster of Malaysia’s governing party this month after six decades in power, Mr. Koh’s family and supporters are calling for a new investigation into his kidnapping.