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Lahore, Pakistan • Pakistani authorities are investigating allegations that cough syrup has killed 33 people over the past three days, a government official said Saturday, the second time in recent months medicine is suspected of causing multiple deaths.

Also Saturday, an explosion ripped through a passenger bus while it was at a terminal in the southern city of Karachi, killing six people and wounding 52 others, police and hospital officials said. It's unclear if the blast was caused by a bomb or a gas canister exploding.

The deaths from the cough syrup occurred in the eastern city of Gujranwala and nearby villages, said local official Abdul Jabbar Shaheen. Another 54 people thought to have consumed the syrup are also being treated at city hospitals. Officials believe the victims drank the syrup to get high, he added.

Tests show the victims' stomachs contained dextromethorphan, a synthetic morphine derivative used in cough syrup that can have mind-altering effects if consumed in large quantities, said Shaheen. Investigators are trying to determine if the victims drank too much syrup, or whether there was a problem with the medicine itself, he said.

Twenty-three people died in the nearby city of Lahore in November after drinking bad cough syrup sold under the brand name Tyno. They were also described at the time as people who consumed the drug to get high.

Shaheen said the cough syrup involved in the incidents in and around Gujranwala was not sold under a single brand. He said some people in the city make cough syrup to sell specifically to drug addicts, and officials are trying to arrest them.

Officials temporarily closed one Lahore-based pharmaceutical company whose cough syrup was found in the possession of some affected in Gujranwala. They are investigating whether it caused any of the deaths, said Shaheen.

The blast that ripped through the bus in Karachi on Saturday set the vehicle on fire and reduced it to little more than a charred skeleton. Police were trying to determine whether the explosion was caused by a bomb or a gas canister, said police spokesman Imran Shaukat. Many buses in Pakistan run on natural gas.

The explosion killed six people and wounded 52 others, some of whom were in critical condition, said Seemi Jamali, a doctor at the hospital in Karachi where they were being treated.

Karachi has a long history of political, ethnic and sectarian violence. It is also believed to be home to many Taliban militants who have fled U.S. drone attacks and Pakistani army operations in the country's northwest.

Also Saturday, gunmen opened fire on a vehicle carrying police officers in the southwestern city of Quetta, killing three of them, senior police officer Hamid Shakil said. No one claimed responsibility but authorities have blamed local insurgents for such previous attacks.

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Associated Press writers Adil Jawad and Abdul Sattar contributed to this report from Karachi and Quetta, Pakistan.