Builders and decorators working on a new laboratory at Oxford University have been sent threatening letters by animal rights extremists.
The centre is expected to cost £18m
Police think the letters form part of a campaign to stop work on the new animal research facility at South Parks Road.
Detectives are investigating 18 anonymous letters sent to companies across Oxfordshire threatening action from the Animal Liberation Front.
They also believe other people may have received letters but not reported it.
A Thames Valley Police spokesman said: "It is very important that anyone who is sent a threatening letter, or becomes the victim of any other harassing or intimidating behaviour, lets us know what has happened.
"Although everyone is entitled to an opinion about this very emotive issue, it is just not acceptable to act in a way which intimidates other people and threatens their livelihood.
"Thames Valley Police has worked with representatives from groups who wish to protest about the building and the university to facilitate a regular time and place for active protest.
"Criminal activity will not be tolerated from any section of the community."
Last year the university took out an injunction preventing protestors from demonstrating too near the site.