Detectives are trying to find out why two bombs found at an Oxford University college on Monday did not explode.
Police are remaining tight-lipped about the content of the two incendiary devices discovered underneath a temporary building at Templeton College in Kennington.
But a spokesman said last night: "The devices look viable and the exact reason they did not ignite will form part of the forensic examination."
The bombs were discovered after Oxford University security staff saw a posting on the Animal Liberation Front's website - Biteback - on Sunday.
Police have still not confirmed that the ALF was behind the attack but said that animal rights was a strong line of enquiry.
A special police operation was established to monitor and investigate attacks against Oxford University by animal rights extremists and a police spokesman said the site was regularly monitored.
However, the police did not comment on why the posting had not been picked up before the member of university staff noticed it.
All items seized at the scene have been sent away for testing at a forensic laboratory.
The police spokesman added: "We would ask members of the public, particularly those who are part of the university community, to remain vigilant and to pass on any information to the police."