Saudi authorities arrested 15 people on Tuesday in connection with a shooting in the east of the kingdom the previous night that left five people dead and was apparently aimed at the country's Shiite minority.
The attack happened late Monday evening in the village of al-Dalwah, which is located in the country's eastern al-Ahsa region, a major oil-producing area that is also one of the main centers of the minority Shiite community in this Sunni-ruled OPEC giant.
The attackers shot the victims with pistols and machine guns, according to the official Saudi Press Agency. The agency provided no further details on the identities of the attackers or the victims, and said an investigation is underway.
Saudi Arabia's embassy in Washington said 15 men were arrested in six different cities on terrorism charges in connection with the shootings. The embassy said authorities initially arrested six people and after further investigation, arrested another nine.
The embassy said two members of the Saudi security forces and two suspected gunmen were killed in a shootout in Buraidah in the Qassim region in a security operation linked to the al-Ahsa shooting.
Witnesses said the attack happened inside a Shiite mosque as worshippers were marking Ashoura, which commemorates the 7th-century death of Imam Hussein, a grandson of the Prophet Muhammad who is revered by Shiites. Sunni militants in neighboring Iraq frequently target Shiites during the holiday.
The witnesses spoke on condition of anonymity because of fear of reprisals.
Amateur video posted online purported to show the inside of the Shiite mosque, called Husseiniya. Pools of blood could be seen on the floor and a witness held several spent rifle shell casings.
"There is blood everywhere. They entered inside the Husseiniya and started to open fire with rifles," said one of the people in the video.
The authenticity of the video could not be independently confirmed but it was consistent with The Associated Press' reporting of the incident.
Saudi Arabia's Shiite minority has long complained of discrimination. Many in the ultraconservative Saudi Wahhabi school of Islam view Shiites as heretics, and the government is suspicious of nearby Shiite powerhouse Iran.
The attack comes weeks after a court in the kingdom convicted a highly revered Shiite cleric of sedition and other charges and sentenced him to death. The cleric, Sheik Nimr al-Nimr, has been critical of the government and was a key leader of 2011 Arab Spring-inspired Shiite protests in the east of the country.
Associated Press writers Adam Schreck in Dubai, United Arab Emirates and Matthew Lee in Washington, contributed reporting.