A bomb exploded Sunday at a Shiite mosque in the eastern city of Lahore, killing at least four people and injuring others, officials said. A witness said a man carrying a briefcase tried to enter the Husainia Hall mosque in the ancient walled part of the city during evening prayers but was blocked by security guards. He then detonated a bomb in the briefcase. "Our two security guards were martyred and the suicide bomber was killed," said witness Sajjad Bhutta.The state news agency quoted Punjab province Home Secretary Hassan Waseem as saying that four people were killed and others injured. The bomb went off at the mosque at 5:40 p.m. after prayers, said Raja Basharat Illahi, the province's law minister. The attack was the third this month against a religious target in Punjab, stoking fears of spiraling sectarian violence. On Oct. 1, a suicide bombing at a Shiite mosque killed 31 people in Sialkot city. Six days later, a car bombing at a gathering of Sunni Muslim radicals in Multan city killed 40 people. No group claimed responsibility for those attacks, which prompted authorities to ban religious gatherings across the country -- except for Friday prayers at mosques.