Gunmen armed with automatic rifles and pistols stormed a Bahraini prison Sunday, killing a police officer and freeing 10 inmates convicted on terrorism charges, police said.

The attack on Jaw prison represents a significant escalation of the simmering unrest that has gripped the island nation off the coast of Saudi Arabia since its 2011 Arab Spring protests. No group immediately claimed responsibility for the assault.

The attack sparked a manhunt across the island Sunday. Confusion about the escape at Jaw prison persisted for hours afterward, as the Interior Ministry issued a series of tweets in Arabic and English contradicting each other.

Early Monday morning, the Interior Ministry issued a statement saying the attack was launched by "a terrorist cell of four to six members armed with automatic rifles and pistols." It named the dead police officer as Abdulsalam Saif Ahmed and said a second officer suffered moderate wounds in the attack.

Bahraini residents described stepped-up checkpoints across the country on Sunday, a day that also saw clashes between police and locals in the community of Sitra.

Bahrain, a small island off the coast of Saudi Arabia, hosts the U.S. Navy's 5th Fleet and an under-construction British naval base.

The nation has seen low-level unrest since its 2011 Arab Spring protests, which saw the nation's Shiite majority and others demand more political freedoms from the country's Sunni rulers. The kingdom launched a wide-ranging crackdown on dissent in April, imprisoning some prominent political figures and forcing others into exile.

Activists, including imprisoned human rights advocate Nabeel Rajab, have alleged that Jaw has been the site of prisoner abuse.