Messages passing through the BlackBerry Messenger system are almost certainly already under examination by the police, who need neither warrants nor ministerial permission to search them for evidence.

While the Regulatory Investigatory Powers Act (RIPA) is necessary for interception of live communications. once the messages have been sent the archives (if archives there are) come under the Data Protection Act. The DPA is a much more flexible bit of legislation, which could permit the police to trawl all the messages sent over the network for evidence identifying individuals.

That's the opinion of Kathryn Wynn of Pinsent Masons, who told us that RIM could, potentially, hand over the entire archive of messages sent over the last few days. That would allow the police to search for keywords and discard those messages in which they weren't interested, though the police could equally ask RIM to do the searching for them.

BlackBerry Messenger is a closed system, messages are only sent between previously identified parties and aren't normally readable by anyone else. Unlike the public forum of Twitter, BBM messages generally remain private, more like SMS with group-send than a social network.

Also like SMS, but unlike most instant messaging systems, all BBM messages go through RIM's UK servers, providing an ideal opportunity for archiving and intercepting such communications.

We don't know, for certain, that RIM has an archive of instant messages sent, but there's good reason to think it does. UK mobile network operators store the historical location of every mobile phone for a year, along with text messages and call records, to o comply with RIPA requests from the authorities, so it seems probable RIM is bound by the same legislation.

When we asked RIM about this, the company provided the following statement: "Similar to other technology providers we comply with the Regulation of Investigatory Powers Act and co-operate fully with the Home Office and UK police forces" – so one can be reasonably certain that even if it wasn't logging everything before, it is now.

Once logged that data comes under the Data Protection Act, rather than the RIPA, so police can use "proportionate" steps for the detection of a crime. RIM could, for example, provide the entire archive to the police to search without the identities of the users attached, allowing the police to check for interesting messages and then ask the identities of the senders. But it would probably be easier for the police to just ask RIM to carry out the same search and provide the results.

We asked the Information Commissioner's Office, custodians of the DPA, about this and were told that the police have a raft of powers to access the archived information and that the ICO probably won't be involved.

That won't help the police know what's happening now: real time interception would still come under the RIPA, but it will help the police to track down anyone they think is inciting to riot, and the Metropolitan force has made it clear it will be pursuing convictions for such things.

The role of BBM in the riots is still contested, with some pointing out that the sound of police overhead, rushing fire engines, and streaming news from Sky, will help lost looters more than instant messaging, but that's not stopped calls for the service to be shut down for a couple of days in the hope of preventing more coordinated gatherings.

That's unlikely to happen, despite a Twitter campaign backed by BSkyB, but anyone imagining that BBM traffic is in some way beyond the law enforcement community is badly mistaken. ®