New Delhi, April 4 -- A man detained in Bahrain last month as the island kingdom came under missile attack from Iran vanished for days, until his family was called to retrieve his body from a military hospital.

Relatives said Mohamed al-Mousawi, a Shiite Muslim who had previously been imprisoned, was saving money to start a business. His body was returned covered in slash marks and bruising, including on the soles of his feet.

His death has become a flashpoint in the Sunni-ruled, Shiite-majority country on the war's front lines, where critics say authorities have revived tactics used to suppress Arab Spring protests in 2011.

Bahrain, a monarchy that hosts the U.S. Navy's 5th Fleet, has arrested dozens of people throughout the war for f...