From the BBC:

Police are to investigate whether an MI5 officer was complicit in the torture of ex-Guantanamo detainee Binyam Mohamed. ...

Ethiopian-born Mr Mohamed says he was tortured while in US custody in Pakistan, Morocco and Afghanistan, with the complicity of MI5.'

He says that in Morocco in 2002, he was mistreated by local officers who asked him questions supplied by British intelligence.

Mr Mohamed returned to the UK in February 2009 after seven years in custody - four of which were spent in the US camp at Guantanamo Bay.

BBC security correspondent Frank Gardner said the investigation would focus on an MI5 officer known as Witness B, who travelled to Karachi in 2002 to question Mr Mohamed.

It is alleged that Witness B told Mr Mohamed that his only way out of Pakistani custody was to co-operate fully, and that the officer knew to where Mr Mohamed would be subsequently rendered.

Our correspondent added that a police investigation would be "deeply uncomfortable" for many in MI5.