Rescue workers in the northeastern Pakistani city of Lahore struggled into the night to reach dozens of people still trapped in the ruins of a factory that collapsed early Monday following an explosion, killing at least 11 people and injuring scores of others.

Police said they believe the three-story factory, which produced veterinary medicines, came crashing down while about 60 people were inside the building. Several young children were reported to be among the victims.

Local authorities said a gas cylinder of a boiler may have caused the explosion. They ordered an investigation.

Emergency workers have rescued at least 13 people, but they say their efforts have been hampered by the narrow streets around the factory, slowing the arrival of heavy machinery.

Authorities say the factory had been ordered closed. It was not immediately clear why it was operating despite the closure order.