The first pictures of the letter bombs used to target businesses across the country have been released by police.

The images show one unexploded device, which was intercepted, still in an A5 padded envelope. The second is of the remains of a bomb which ignited and caused minor injuries.

Details including postmarks and handwriting have been deliberately covered up by detectives. They say this is essential, so as not to compromise the investigation.

Nine people have been injured after opening packages sent to companies including the Driver and Vehicle Licensing Agency (DVLA) in Swansea, forensic science laboratories in the West Midlands and to Capita, which runs London's congestion charging system.

On Thursday, a 48-year-old man was arrested and detained under the Mental Health Act after he tried to claim responsibility for sending one of the bombs.

Forensic teams are searching two residential properties in Bracknell and Wokingham in Berkshire in connection with the arrest. But police say they are still investigating other leads and examining several theories, including the possibility that the attacks are the work of animal rights extremists.

The names of different protesters were featured on at least two of the packages, including that of Barry Horne who died while in jail. However, animal rights groups have reacted angrily to suggestions that they are responsible for the letter bombings.

Greg Avery, who founded a campaign to force the closure of the animal-testing company Huntingdon Life Sciences, said that the letter bomber was trying to mislead police in an attempt to draw attention away from themselves.

"The police are just trying to smear us [the animal rights movement]. If an activist was going to risk a lot of time in jail they would not be sending parcels to the DVLA."