A woman was injured when animal rights extremists sent letter bombs to two Oxfordshire science firms yesterday.

The woman was hurt 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.

And four-and-a-half hours later police were called to LGC Forensics, where a second device had been spotted before it could go off, because staff were on high alert following the first attack.

Detectives have linked both bombs and believe they were sent through the mail by animal rights extremists.

But last night they admitted they did not know why either company was attacked.

There is not an immediately apparent link to Oxford University, which is building an £18m animal experiments laboratory in South Parks Road.

The attacks led to a nationwide alert being made to all firms Government officials believe are potential targets.

The letter to Orchid bore the sender's name of Dr Barry Horne, a well-known animal rights activist who died on hunger strike in prison in 2001.

The second letter carried a different name, which has not been released, "but it does lead to a link to animal rights extremists," a police spokesman said. She added: "We absolutely condemn these attacks.

"These are obviously serious incidents which we will investigate thoroughly.

"They could have led to serious injury."

The woman victim of the letter bomb suffered a minor injury to her hand, but, add the police spokesman: "It was lucky she was not seriously hurt.

"There have been similar incidents involving the post which have subsequently proved to have an animal rights connection."

She said she did not know why either firm had been attacked by extremists but added: "We will build up the links from there."

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.

The squad also "made safe" the second package at LGC Forensics - formerly known as Forensic Alliance - which does a lot of the scientific investigative work for Thames Valley Police.

Last night a senior police officer was speaking to the bosses of both companies.

The construction of the new animal testing laboratory has prompted several attacks on businesses and sports facilities by animal rights activists. Police put the number at about 20.

A spokesman for LGC Forensics said: "We did receive a suspect package through the post. It was identified at an early stage and authorities were alerted.

"When we heard the news about Orchid Cellmark we were being extra vigilant about all packages. It has been that sort of a day."

Robin Webb, a spokesman for the Animal Liberation Front, was unavailable for comment, and the ALF has not yet claimed responsibility for the letter bomb on its Biteback website.

An Oxford University spokesman declined to comment on any possible links between Orchid Cellmark and the university.