Two Saudi policemen were shot dead on Wednesday when they came under fire in the kingdom's east, where most of the minority Shiite community live, the interior ministry said.
The pre-dawn attack happened near a farm in the Saihat area of Qatif, on the Gulf coast.
A ministry spokesman said the suspect or suspects in the "terrorist crime" had not immediately been identified.
Hussein al-Nemr, who lives in the area, said the early morning shooting happened near a highway checkpoint.
He passed it later but could see no signs of what happened.
It is the latest incident in Eastern Province as police continue to hunt suspects on a list of 23 wanted people issued in 2011 after Shiites began protests.
Shiites in the Sunni-dominated kingdom have complained of marginalisation.
Last week, two Indian passersby and a policeman were wounded when officers came under fire from a farming area in Qatif.
Many suspects on the wanted list have already been detained or killed in shootouts.
In early October, gunfire erupted in the Shiite town of Awamiya, near the Gulf coast city of Dammam, after police sealed it off with armoured vehicles.
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Police seized a large amount of ammunition during a search of a farming area there, the interior ministry said.
Last month, Saudi Arabia's Supreme Court confirmed a death sentence against Shiite cleric Nimr al-Nimr, a leader of anti-government protests convicted of sedition and other crimes.
Activists are calling for the sentence not to be carried out, saying Nimr engaged only in peaceful opposition.
He is one of seven Shiites sentenced to death in connection with Shiite unrest since 2011, activists say. Three of the convicts were under the age of 18 at the time of their arrests.
Over the past year, Saudi Shiites have been targeted by suicide bombings and shootings claimed by Sunni extremists of the Islamic State group, who consider them heretics.
In the latest attack, a gunman killed five Shiites in Saihat last month during commemorations of Ashura, one of the holiest observances of their faith.
In June, four Shiites died preventing a suicide bomber from entering the hall of Al-Anoud mosque in Dammam city.
Days earlier, 21 people were killed in another Shiite mosque bombing in Eastern Province.
Nemr, the Qatif resident who is not related to the detained sheikh, told AFP that Shiites are worried.
If the deaths of the two policemen are linked to the hunt for suspects on the wanted list, then there could be a crackdown.
But if it were an attempt at IS infiltration, people fear it could indicate plans for another deadly assault on Shiites, he said.
"We hope (it was) neither," Nemr said, noting that on December 2, Shiites will mark Arbaeen, the death of Imam Hussein, one of the most revered figures of their faith.