A woman who was beaten to death in northern Nigeria on suspicion of being a suicide bomber was in fact mentally ill and not involved in terrorism, according to police and her family.

Thabita Haruna, 33, was attacked by a mob on Sunday after she refused to be screened at a marketplace in Bauchi.

Police are investigating the attack and have yet to make any arrests.

Boko Haram militants based in northern Nigeria have been using women to carry out suicide bombings.

The militants want to impose a strict version of Islam and have declared a caliphate in the territory under their control.

Ms Haruna was set upon by a mob last Sunday after she refused to be searched by vigilantes at the entrance to a marketplace.

A witness told the AFP news agency that the woman came under suspicion when two bottles were found strapped to her waist.

The mob placed a tyre sprinkled with petrol over the woman's head and set it alight after she was beaten, according to witnesses.

Police say that she was dead by the time they arrived at the scene. Earlier reports had indicated that the woman was a teenager.

The woman's family says she had worked as a market trader until 2007, when she became mentally ill.

"I feel very very sad because she is my blood," Ms Haruna's sister told the BBC this week. "We sleep in the same bed, we eat at the same plate…. That really pains me."

According to the BBC Hausa service's Ishaq Khalid in Bauchi, the people in the town have condemned the attack.

Boko Haram has taken to sending women on suicide missions, fuelling concern that its insurgency has entered a more ruthless phase.

Teenagers have been used to carry explosives into busy markets and bus stations, raising additional fears that some of Boko Haram's hundreds of kidnap victims may have been forced into carrying out bomb attacks.

February's presidential election has been postponed because of the unrest. The vote is now due to take place on 28 March.