A primary school caretaker has been arrested in connection with a series of letter bombings that saw a woman injured at an Abingdon firm.
The woman was hurt last month when the device exploded as she opened the morning mail at Orchid Cellmark, a bioscience firm specialising in DNA work, in Blacklands Way at Abingdon Business Park.
Four-and-a-half hours later, police were called to LGC Forensics, in Culham, where a second device had been spotted before it could go off.
Orchid Cellmark was evacuated as the bomb squad from the Army's Royal Logistic Corp was called in to deal with the package after it went off at 9.15am. The firm was cordoned off until 1pm.
Another letter bomb went off in the West Midlands.
The attacks became part of a major police probe after four similar devices also went off at locations around the country, including the London offices of Capita, which runs London's congestion charge system, and at the DVLA in Swansea.
The police initially indicated that both animal rights extremism and the possibility of a motorist with a grudge were being examined.
A school caretaker in his 20s was arrested today over the attacks.
Police went to a house in Cherry Hinton, near Cambridge, at about 3am to arrest the suspect.
He was named by sources as Miles Cooper.
Specialist forensic teams began a search of the property that is expected to last for at least two or three days.
Police also searched Teversham Primary School, where he works, with the co-operation of the school's head teacher and Cambridgeshire County Council.
Next-door neighbour Keith Bailey said Mr Cooper had lived at the house with his mother Lorraine and sister Sally, who is also in her 20s, for nearly 20 years.
"I see Miles most mornings when I'm out walking the dog," said Mr Bailey, 65, a retired paint stripper.
"He's usually out walking, although I don't know where he would be walking to.
"Mostly all we do is say hello. That's about all you ever get from Miles, he is a very quiet sort of boy."
Sources close to the investigation described the arrest as a "very significant" development.
Anton Setchell, the Association of Chief Police Officers' national co-ordinator for domestic extremism, said: "At this stage I'm not able to guarantee that there is not another postal package containing an explosive device within the postal system."
The man is being held in connection with all seven letter bombings, which police now believe are linked. At least nine people have been injured in the blasts.
In each of the attacks, the bombs have been home-made pyrotechnic-style devices in A5 padded envelopes. At least two contained glass and at least one bore a Cambridgeshire postmark.