A primary school caretaker has been arrested in connection with the recent spate of letter bomb attacks.
Police outside the suspect's house in Cambridge
Miles Cooper, who is in his 20s, was arrested at a house in Cherry Hinton, near Cambridge.
Police will finish searching the school where he works on Tuesday and expect to spend several days searching the house.
Seven letter bombs were sent over a period of three weeks and the police have warned that they cannot guarantee that there will not be another one.
Nine people have been injured by the letter bombs, including four workers who were hurt when a bomb exploded at the Driver and Vehicle Licensing Authority centre in Swansea.
Anton Setchell, the Association of Chief Police Officers' national coordinator for domestic extremism, said a man was arrested in the early hours of Monday morning and was being held at an undisclosed police station.
Mr Cooper's home, in Welstead Road, has been cordoned off and according to Mr Setchell, forensic teams are conducting a "lengthy search", which will last for "several days".
Mr Cooper is the caretaker at Teversham Church of England primary school at Teversham, near Cambridge, which is also being searched.
Mr Setchell said the risk of anything dangerous being found at the school was incredibly low but he wanted to ensure it was safe for children and staff.
Police expect to complete the search at the school on Tuesday.
Detectives have been working with Cambridgeshire police after a parcel bomb was sent to a Labour Party office in Cambridge in August.
At least one of the recent bomb packages is said to have had a postmark from Cambridgeshire.
MAIL BOMB LOCATIONS
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
Mr Setchell said: "We have now reached a very significant stage in this inquiry."
But he added: "At this stage I am not able to guarantee that there is not another postal package containing an explosive device within the postal system.
"The previous seven devices have all been contained in A5 size jiffy-type padded envelopes.
"I am therefore renewing my request for the public to maintain their vigilance and not to handle any post which appears in any way suspicious.
"If anyone is concerned about any package that arrives in the post they should contact their local police immediately."
Next-door neighbour Keith Bailey said Mr Cooper had lived at the house with his mother Lorraine and sister Sally, who is in her 20s, for the best part of 20 years.
Mr Bailey, 65, said: "Mostly all we do is say hello. That's about all you ever get from Miles, he is a very quiet sort of boy.
"He is the last person in the world I would have thought would be arrested over something like this."
He added: "He is not a rowdy type of lad at all. Very quiet; you don't see him about much.
"They don't have lots of young friends around in noisy cars or anything like that. They are very quiet."
The bombs have been home-made pyrotechnic-style devices with at least two containing glass.
And two of the parcels contained names of animal rights protesters, including Barry Horne who was jailed and died while on hunger strike in 2001.