Bahrain's Shiite opposition party said Wednesday that another one of its supporters, the fourth to date, had died in police custody.
Al Wefaq, Bahrain's main opposition party in the Sunni-ruled Gulf country, said Haji Karim Fakhrawi died in "mysterious circumstances," while his relatives pointed to a body covered in bruises saying he had died of torture.
Bahrain's official news agency cited the medical examiner from the army hospital saying Fakhrawi died of kidney failure.
At least 30 people have been killed since anti-government protests by Bahrain's Shiite majority began in February. Hundreds have been detained in the crackdown on the rebellion.
State Department spokesman Mark Toner said the U.S. was aware of Fakhrawi's death and was deeply concerned about the deaths of individuals while in custody in Bahrain.
"We strongly urge the government of Bahrain to ensure the security of all arrestees," Toner told reporters in Washington.
He said Bahrain's government must guarantee transparent judicial proceedings in line with Bahraini and international standards. It should also conduct complete and transparent investigations into all the deaths, he said.