A top security official in Bahrain says four suspects have been arrested in connection with bombings that killed two people and brought warnings of increased crackdowns against opposition groups in the violence-wracked Gulf nation.

A series of blasts Monday in the capital Manama killed workers from India and Bangladesh and suggested a widening campaign of attacks by anti-government forces in the nearly 21-month uprising against the Western-backed monarchy.

Bahrain's chief of public security, Maj. Gen. Tariq al-Hassan, said Tuesday that four suspects were arrested but noted the investigation is not over.

More than 55 people have died in Bahrain's unrest since February 2011 after the country's Shiite majority escalated its push for a greater political voice in the strategic Sunni-ruled nation.

Bahrain is home to the U.S. Navy's 5th Fleet.