MOGADISHU (Reuters) - A blind suicide bomber killed Mogadishu's mayor Abdul Rahman Omar Osman and six others last month and was a local government employee in the capital and received help from a colleague, the Somali Ministry of Internal Security said on Monday.

Al-Qaeda-linked al Shabaab, which seeks to topple the UN-backed Somali government, has claimed responsibility for the July 24 terrorist attack in the capital.

A week after his injury, Mogadishu Mayor Abdul Rahman Omar Osman died in Doha, where he was taken for treatment.

"Preliminary results of the investigation show that a woman who worked in Mogadishu's local government blew herself up with the help of another woman," the Department of Homeland Security said in a statement.

It was the first time that al Shabaab was reported to have employed a disabled person in a suicide bombing. The statement noted that the woman took advantage of her disability to pass through security and access to the mayor's office where the attack took place.

The whereabouts of the woman, who the Department of Homeland Security statement said helped her carry out the attack, were not known.

The ministry said the two women had been given work leave a month before the attack and traveled to an area controlled by al Shabaab.