LONDON - Opposition politicians questioned Thursday why the prime minister did not require his former communications director to obtain a top-level security clearance that could have turned up some evidence that eventually led to his resignation and arrest over phone-hacking and bribery.

Andy Coulson is accused of overseeing hacking and payoffs to police officers while serving as editor of the News of the World. He left that job over the allegations, but went to become David Cameron's communications chief.

The scandal grew after British police reopened the probe into the phone hacking at the newspaper and Coulson resigned in January. He was arrested on July 8 as the scandal forced Rupert Murdoch's News International to close down News of the World, saw two senior police officials resign and brought to light links the company had with the police and British politicians.

The Guardian reported Thursday that Coulson had not been required to obtain top-secret clearance, the highest level of security vetting, which allows the holder to see high-level national security documents.

Alistair Campbell, Tony Blair's communications director, and others who held the post in recent Labour governments had the higher-level top secret clearance.

The Cabinet Office, which oversees the civil service, said all employees in the prime minister's office have at least the second-highest level of clearance. They would be subject to a check of company records, credit and MI5 records. An interview can be required if any security concerns are unresolved.

A spokeswoman at Cameron's office said the level of vetting depended on whether someone had access to top secret material. She declined to comment on why Coulson didn't have that level of clearance.

"We don't comment on individual vetting," she said, speaking on condition of anonymity in line with government policy.

Cameron was asked several times about the issue in the House of Commons on Wednesday.

"He had a basic level of vetting," Cameron said. "He was not able to see the most secret documents in the government."

When the question was raised again, Cameron said, "I feel that a number of honourable members are looking for some sort of secret behind a curtain that simply is not there."

Helen Goodman, one of the opposition Labour Party members, said "one can but speculate" why Coulson would not have been subjected to the highest level of vetting.

"Your guess about the prime minister's motives is as good as mine, but I certainly haven't had clear answers from him so far," Goodman told The Associated Press.

Meanwhile, a member of the board that oversees London's police has asked the force to investigate claims that News of the World reporters paid officers to locate people by tracking their cellphones, a practice known as "pinging."

Jenny Jones, a Green Party member of the Metropolitan Police Authority, cited claims by a former News of the World reporter that mobile phone-tracking data could be purchased for 300 pounds ($480).

The allegation was made by Sean Hoare, a former News of the World reporter who spoke to the New York Times about skullduggery at the tabloid. He was found dead on Monday at his home near London; police say the death is not suspicious.

The hacking scandal has received feverish attention since July 4 when it was revealed that someone at News of the World had hacked the phone of 13-year-old murder victim Milly Dowler at a time when police were still searching for her.

The temperature cooled a bit on Thursday, with Parliament closed for the first day of its summer recess, but the investigation appeared to be intensifying.

London's Metropolitan Police said Wednesday that it was assigning 15 more officers to help the 45 already involved in the investigation.

News Corp., meanwhile, said it had instructed the law firm of Harbottle and Lewis to answer police questions about emails and other documents from an internal investigation at News of the World in 2007. That inquiry said found no evidence that Coulson was aware of hacking by reporter Clive Goodman and private investigator Glenn Mulcaire. Both were sentenced to prison for hacking into phones of the royal household.

Harbottle and Lewis had said there was no evidence of wider criminality at the newspaper.

The file of emails and document was turned over to police in June.

Mulcaire said in a short statement released through his lawyer that he will not speak out about his involvement in the phone hacking scandal until the police inquiry is completed.

Ken MacDonald, the former director of public prosecutions, reviewed emails from that file which related to payments to police. He had been hired to advise the News Corp. board.

On Tuesday, MacDonald told a parliamentary committee that it took no more than five minutes to read the material. "I cannot imagine anyone looking at that file and not seeing evidence of crime on its face," MacDonald said.

Since the Milly Dowler hacking was reported, London's police chief and the head of its anti-terrorist operations have resigned; so have Rebekah Brooks, chief executive of News International, which runs Murdoch's British papers, and Les Hinton, a longtime associate of Murdoch who formerly headed News International. The News of the World was shut down, leaving 200 employees looking for work, and the BSkyB bid was shelved.

Shutting News of the World apparently will cost Rupert Murdoch's surviving British newspapers their exclusive access to British athletes ahead of the 2012 London Olympics.

Team 2012, an initiative supporting British Olympians, had signed up News International as its official partner to help raise funds for athletes. But without News of the World, Team 2012 said News International can no longer meet its contractual obligations, and it is looking for new media partners.