At least 41 people have been killed and many more have been wounded in an attack on a bus in the Pakistani city of Karachi, officials told Al Jazeera.

The bus belonged to the Ismaili Shia Muslim sect, and was targeted by gunmen at the Safoora Chowk intersection in the eastern part of Pakistan’s largest city on Wednesday.

Sindh police chief Ghulam Haider Jamali told local media that initial reports suggest there were six attackers, who approached the bus on three motorcycles.

He said that those killed had been hit by 9mm gunfire, suggesting that handguns had been used in the attack.

At least 20 people were wounded, and many of them were in critical condition, hospital official Salma Wahid told Al Jazeera.

“Their condition is serious and they were covered in blood when they came in,” said Wahid, adding that many were unconscious when they were brought to the Memon Medical Institute and Hospital.

Sindh Chief Minister Qaim Ali Shah, who heads the provincial government, said that he had ordered senior police officials to investigate the incident.

“I have taken it very seriously. I am terribly sorry that this nasty incident has taken place. Whoever it is, we have to detect the offence and take action against the culprits,” he told local media shortly after the attack.

This is the fifth major attack against Shia Muslims in Pakistan this year, with previous attacks including suicide attacks and bombings at Shia mosques in Rawalpindi, Islamabad, Peshawar and Shikarpur. Those attacks claimed the lives of at least 98 people.

Karachi, home to at least 20 million people, has often seen incidents of targeted attacks on political, ethnic and sectarian grounds, although violence has decreased since a paramilitary operation against criminals was launched in September 2013.