A school caretaker was being questioned last night over a letter-bomb campaign that has injured ten people.

Police gave warning that other explosive devices may already be in the post.

Miles Cooper, 26, was arrested at 3am yesterday by detectives investigating seven explosive devices sent in Jiffy-type envelopes to companies linked to motoring enforcement and the “surveillance society”.

Scenes-of-crime officers yesterday searched Teversham C of E Primary School, in Cherry Hinton, near Cambridge, where Mr Cooper has worked as a caretaker and cleaner for the past two years.

Mr Cooper, described by neighbours as a bicycling loner, was arrested at his home in Cherry Hinton where police are believed to have had him under surveillance for several days. Officers removed computer equipment and said that they expected to spend several days searching the three-bedroom home.

They also searched the home of his father, Clive, in March, Cambridgeshire, paying particular attention to the garage.

A police source told The Times: “We had a significant breakthrough in the investigation at the weekend and have had a suspect under surveillance. We believe this is a very significant development.”

Mr Cooper, who lives with his mother and sister, had previously had several jobs working with children and also worked at a Tesco supermarket. Neighbours and colleagues described him as a shy man who cycled everywhere.

Mr Cooper’s uncle, Peter Mepham, 52, said: “He has been a lonely boy all his life.”

A spokesman for Cambridge County Council said that Mr Cooper had passed a full Criminal Records Bureau check when he was employed as a cleaner at Cherry Hinton community junior school in 2002.

When he moved to Teversham school in August 2005 the council carried out a check against the Department for Education’s “list 99” of people barred from working with children. Mr Cooper started work at a nursery at Addenbrooke’s hospital when he left Netherhall Secondary School in Cherry Hinton, A former colleague said: “He used to look after one and two-year-olds but we thought he was better suited to the older children. He was a very quiet guy.”

Anton Setchell, the Association of Chief Police Officers’ national co-ordinator for domestic extremism, said he could not guarantee that there were no other devices already in the postal system. “The previous seven devices have all been contained in A5 size Jiffy-type padded envelopes,” he said.

“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 first of the letter bombs were sent in mid-January. They were directed at forensic science laboratories in the West Midlands and the Oxfordshire area. They were followed by an attack on February 3 on a house in Folkestone, Kent, which is used as a business address of a security company.

The following week three letter bombs exploded on consecutive days at motoring-related companies in London, Wokingham, Berkshire and Swansea. They included the Driver and Vehicle Licensing Agency in Swansea and the offices of the company that runs London’s congestion charging system.

The bombs have been homemade pyrotechnic-style devices and at least two contained glass. At least one was posted from the Cambridge area.

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