"The absence of a police presence or sight of police officers apparently standing by and allowing criminal behaviour to take place was distressing for those whose homes and businesses were being attacked, " it added.

BBC: England riots: Police operation 'flawed' say MPs

However, U.S. officials briefed on the matter said the probe was closely held among officials at the FBI and Justice under a long-standing policy not to divulge information on continuing criminal investigations.

WSJ: Petraeus Affair Puts Scrutiny on FBI

Mr Musharraf is already on bail for several other criminal cases and has been disqualified from standing in the forthcoming election.

BBC: Asia