The name of a second animal rights activist was found on one of a series of letter bombs sent to UK businesses, the BBC has learned.
A letter bomb was sent to Swansea's DVLA centre
The letter was delivered to LGC Forensics, in Culham, Oxfordshire, on 18 January and failed to explode.
Another device carried the name of jailed protester Barry Horne who died on hunger strike in 2001.
A 48-year-old man who claimed he sent one of the bombs has been arrested and detained under the Mental Health Act.
The man "confessed" to sending a bomb to accountancy firm Vantis in Berkshire after calling BBC Radio 2's The Jeremy Vine Show.
He was not allowed on to the live show and the police were contacted immediately.
Police are still checking for activist links to all the letters, the latest which was sent to the DVLA in Swansea.
'Animal rights links'
Thames Valley Police told BBC Radio's Five Live the LGC Forensics letter included the name of a "prominent animal rights activist who is still living".
Police said the name was written on the envelope.
They would not reveal the identity of the protester but said there were strong links between the first three letter bombs received and animal rights activists.
Three letters were received on 18 January, the first by LGC Forensics and the second by Orchard Cellmark, in Abingdon, Oxfordshire, which injured one woman.
A third letter bomb was delivered to Forensic Science Services in Chelmsely Wood, Solihull. That too failed to go off.
It emerged earlier this week that the Orchard Cellmark bomb contained the words "Dr Barry Horne RIP".
Mr Horne, who was not a doctor, died in hospital on his fourth hunger strike in 2001. He had been sentenced to 18 years for planting bombs.
He is seen as a martyr figure among some animal rights activists.
In the Oxfordshire and Birmingham cases, the companies received an A5 jiffy bag containing a crude firework-type explosive device.
1 Forensic Science Service, Chelmsley Wood - 18 Jan 2007
2 Orchid Cellmark, Abingdon, Oxon - 18 Jan 2007
3 LGC Forensics, Culham, nr Abingdon - 18 Jan 2007
4 Private house, Folkestone, Kent - 3 Feb 2007
5 Capita, London - 5 Feb 2007
6 Vantis, Wokingham, Berks - 6 Feb 2007
7 DVLA, Swansea - 7 Feb 2007
A second line of inquiry is that the bombs were sent by a disgruntled motorist.
On Monday, a bomb was received at the central London offices of Capita, responsible for the congestion charging system, which caused minor injuries to a female employee.
The next day a package exploded at the Berkshire offices of Vantis, an accountancy firm linked to Speed Check Services, in Camberley, Surrey, the provider of digital speed cameras to the police.
Two men suffered blast wounds in the explosion.
On Wednesday, four workers were injured when a parcel bomb exploded at the Driver and Vehicle Licensing Authority's main centre in Swansea.
Police are also investigating a letter bomb which injured 53-year-old Mike Wingfield at his home in Folkestone, Kent, on Saturday.
Workers at the post room at Swansea's DVLA began opening mail again on Friday.
The Public and Commercial Services union (PCS) said there was still a risk to its members, but it was happy with the measures taken to keep the staff safe.
Police have advised anyone handling mail in office post rooms to take extra care.