Nigerian authorities say their top suspect in a deadly Christmas Day bombing at a Catholic church in the capital, Abuja, has escaped police custody.

A police statement Tuesday said the suspect, Kabiru Sokoto, was being escorted to another police station outside of Abuja when “suspected gang members” attacked and freed him. The police commissioner who ordered the transfer has been suspended, and police say they are investigating the incident.

Nigerian media also are raising questions about reports that Sokoto's arrest Saturday occurred while he was staying at a state governor's mansion in the capital.

The Christmas Day bombing was one of a series of coordinated attacks that day that killed at least 39 people.

Nigeria has seen escalating violence between Christians and Muslims. President Goodluck Jonathan recently declared a state of emergency in 15 areas as part of his response to the unrest. The president also has deployed extra troops to the north, but attacks have continued.

Authorities blame a group known as Boko Haram for hundreds of deaths in bombings and shootings over the past 18 months. The group has claimed responsibility for several of the attacks, including those on Christmas Day.

The group is said to want wider implementation of sharia, or Islamic law, across Nigeria. It recently warned Christians in Muslim-majority northern Nigeria to leave the area.

U.N. Human Rights chief Navi Pillay last week called on Nigerian political and religious leaders to conduct joint efforts to halt sectarian violence in the country. Pillay said it is especially important for Muslim and Christian leaders to “condemn all violence,” including retaliatory attacks.