A man is held in connection with letter bomb attacks in recent weeks which injured nine people.
A man has been arrested in connection with the recent spate of letter bomb attacks, police said.
The man, in his 20s, was arrested at around 0300 GMT at an address in Cherry Hinton, near Cambridge.
Police have cordoned off the property and forensic teams will spend several days conducting a search.
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 the man was arrested in the early hours of Monday morning and was being held at an undisclosed police station.
The property, in Welstead Road, has been cordoned off and according to Mr Setchell forensic teams are conducting a "lengthy search" of the residential address, which will last for "several days".
Cambridgshire link
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.
Mr Setchell said: "We have now reached a very significant stage in this inquiry."
However, 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."
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.
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