WASHINGTON — The United States voiced concern Wednesday over the deaths of people in police custody in Bahrain, which has cracked down on a wave of Shiite-led anti-government protests.

Bahrain's Shiite Muslim opposition said a fourth detainee, Karim Fakhrawi, has died in Bahraini custody, as Human Rights Watch urged the Gulf kingdom to investigate the deaths of arrested activists.

"We are deeply concerned by the deaths of both him and several other individuals while in custody of the Bahraini authorities," State Department spokesman Mark Toner told reporters.

He offered condolences to the families of all those who have lost their lives.

"We'd strongly urge the government of Bahrain to ensure the .... security of all arrestees and abide by its commitment to transparent judicial proceedings conducted in full accordance with Bahraini law and Bahrain's international legal obligations," he said.

"And we would call on the government of Bahrain to conduct complete and transparent investigations into all these deaths," he added.

The main Shiite opposition group, Al-Wefaq, said the death of Fawakhri, arrested on April 3, was the fourth since the Sunni-ruled country crushed Shiite-led street protests in mid-March, with the backing of forces from neighboring Gulf states.

The latest reported death has not been officially confirmed.