Police have asked mobile phone and Internet companies to store the content of voicemails, Emails and SMS text messages that were in their systems on the day of the London bombings, a police source said on Sunday.
He said it was only the second time they had issued such a request. The first was in the wake of the September 11 attacks on the United States in 2001.
But police have no legal authority to force the companies to store the information, and some have said they cannot meet the request for technical reasons, the source said.
"The idea is to stop stuff that's going to be important to the investigation disappearing," he told Reuters.
"Over the next few weeks, as the Metropolitan Police identify suspects, they'll want to investigate who they've been communicating with and so on. So the action is to make sure that data isn't deleted routinely by the companies over that intervening period."
Such information normally vanishes from the systems of telecoms companies as soon as users have listened to their voicemail messages or download their Emails, the source said.
This meant that companies which complied with the request would probably be able to retrieve the content of messages only from last Wednesday and Thursday.
More than 50 people were killed in Thursday's attacks by suspected al Qaeda bombers on three London underground trains and a bus, the deadliest militant attack on the city.
A spokesman for BT Group confirmed the company had received the police request, issued on Thursday evening by the National Crime Squad. He declined to comment further, describing it as a security matter.
NEW EU RULES SOUGHT
Earlier, interior minister Charles Clarke said London would seek new European Union rules to make telecoms companies store records for much longer showing who their customers are calling and Emailing.
Clarke said he would raise the issue on Wednesday at a meeting in Brussels of European Union interior ministers which he called to discuss a joint response to the bombings.
Unlike the one-off police request for companies to store the actual content of messages from last week, Clarke is seeking longer retention of connection data showing what calls are made between which numbers.
"We believe that telecommunications records, whether of telephones or Emails, which record what calls were made from what number to another number at what time, are of very important use for intelligence," Clarke told the BBC.
"I'm not talking now about the content of any call, but the fact that a call was made. And we believe it's important to get a retention of data, of what calls were made, for some considerable time."
Mobile phone data show not only what numbers are connected to each other via calls, voicemail and text messages, but also the time and the physical location of the parties within the cell network when a given conversation took place.
Connection records held by Internet service providers show which Email users are in contact with each other, but they do not reveal what Websites a customer surfs.
Clarke did not specify how long telephone companies and Internet service providers could be required to store data, although The Observer newspaper said the proposal was for "several years".
Five EU governments -- Britain, Germany, Spain, France and Italy -- agreed in principle last March that the retention period should be raised to one year.
At present, storage rules vary around Europe but records are typically kept for about three months and destroyed after the customer has been invoiced. Retaining them for longer would impose extra costs on telecoms providers.
Privacy groups complain such measures are excessive.
"There's no evidence that that level of national surveillance is warranted. I believe it's disproportionate," said Simon Davies, director of lobby group Privacy International.
"It will result in a very high level of false accusations and unnecessary scrutiny of people's private communications."
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