Motorcycle-riding assassins have gunned down a Saudi diplomat in the Pakistani city of Karachi, four days after a grenade attack on the Saudi consulate there.

The unusual spate of attacks raised questions about whether they were in reprisal for the death of Saudi-born Osama bin Laden or the consequence of regional Sunni-Shia tensions triggered by upheaval in Bahrain.

A senior police officer said the diplomat, named as Hassan al-Khatani and described as a security officer, was shot dead in his car on Monday morning by two men riding a motorbike who fired four shots from a 9mm pistol.

Television pictures showed a luxury sedan with gunshots through its windows. Police said a backup team of assailants rode alongside the killers, indicating a degree of professionalism in the hit.

On Thursday unidentified assailants threw two grenades at the front gate of the consulate, damaging the entrance but injuring nobody.

Attacks on diplomats from Saudi Arabia are rare in Pakistan, thanks to the country’s close relationship with the army and the widespread reverence towards the country as the home of Islam.

“We’ve always had sectarian tensions but rarely an attack on a Saudi diplomat like this,” said defence analyst Ayesha Siddiqa.

But decades-old Shia-Sunni tensions in Karachi have been reignited by turmoil across the Arabian sea in Bahrain, where Saudi Arabia deployed troops last March to help quell an uprising by mostly Shia demonstrators.

Pakistani Shias became angry when it emerged that a private security firm was urgently recruiting hundreds of former soldiers to work for the Bahrain security forces and help with the crackdown.

Newspaper advertisements sought Pakistanis with experience in “security” and “riot control”.

A senior police officer in Karachi told the Guardian the Bahrain connection was considered the most likely motive for the two most recent attacks. But they were investigating whether they may have been in reprisal for the US special force raid that killed Bin Laden on 2 May.

Riyadh stripped Bin Laden of his citizenship in 1994 and has since co-operated closely with American efforts to crack down on al-Qaida, even though private Saudi citizens have been accused of sponsoring his network.

US intelligence is currently examining a trove of computer drives snatched from Bin Laden’s hideout, reportedly containing 2.7 terabytes of data, for further information about al-Qaida’s money pipeline.

A third possibility was that the attacks were linked to local criminal groups, the officer said. In recent years, he said, “some low-level officials at the consulate had been found to be involved in minor criminal activities with local mafias”.

The difficulty of investigating the killing is underscored by the general insecurity in the sprawling port city of 16 million people, where ethnic, political and Islamist militant groups hold sway in pockets of the city that are virtually out-of-bounds to the security services.

Drive-by attackers lobbed two grenades at the Saudi consulate in Pakistan’s largest city of Karachi on Wednesday, in a first possible violent reaction to the US killing of Osama bin Laden.

Officials reported no damage and no casualties after two men on a motorcycle threw the explosives at the heavily fortified building in Clifton, the smartest neighbourhood of Karachi, nine days after bin Laden was killed in Pakistan.

“This was an attack on the Saudi consulate. Two motorcycle riders threw two grenades and fled,” provincial government official Sharfuddin Memon told AFP.

“One exploded and hit the outer wall. The second landed inside and fortunately didn’t explode. It was later defused by bomb disposal,” he said.

“There were no casualties. We are seeing this incident in the present context. It could be a reaction of the Osama incident.” Pakistan has been in the grip of domestic and international crisis since US Navy SEALS flew in, seemingly undetected, from Afghanistan to identify and kill the Saudi-born Al-Qaeda terror mastermind at a suburban compound on May 2.

Pakistanis have expressed horror at the perceived impunity of the raid, furiously asking if their military was too incompetent to know he was living in a garrison city near the capital, or, even worse, conspired to protect him.

But while the killing has not ignited mass protests in the Muslim country, where more than 4,240 people have died in bomb attacks blamed on the Taliban and Al-Qaeda in the last four years, small gatherings have vowed revenge.

“We fear that desperate elements are planning to launch a big attack. We are taking precautionary measures in this regard,” Memon said.

Saudi Arabia expelled bin Laden in 1991 and later revoked his nationality. The government in Riyadh, which is allied to the authorities in Islamabad, last week welcomed his killing as a boost to international anti-terror efforts.

An AFP photographer said ambulances, police and paramilitary Rangers swarmed outside the Saudi consulate after the attack, where small shrapnel marks could be seen on the outer wall of the building.

Mohammed Safdar, a police official at the scene, said security guards at the diplomatic mission had opened fire on the attackers but they escaped.

“Two security men at the gate opened fire on them, but they managed to flee,” he told AFP.

“The security guards informed us and we reached the spot immediately. The bomb disposal squad are here. Other police and Rangers have surrounded the area,” he added.

Pakistan is holding in protective custody three of bin Laden’s widows, who come from Saudi Arabia and Yemen, and 13 of their children.

The foreign ministry says it has yet to receive a formal request from the United States for access to the relatives or requests from their home countries for their repatriation.