Two letterbombs have been delivered to science firms in Oxfordshire today.
A woman was hurt when a package exploded as she opened the post at Orchid Cellmark, in Blacklands Way, at Abingdon Business Park, at 9.15am.
And at 1.45pm police were called to another firm in Culham where another suspicious package was found.
The device was currently being dealt with, said police.
Both packages are being linked to animal rights protesters.
The Orchid Cellmark employee suffered an injury to her hand. She was treated at the minor injuries unit at Abingdon Hospital.
A police spokesman said: "The device has been sent for forensic examination but a reference to a well known animal rights extremist, Barry Horne, on the back of the envelope, leads officers to make this a priority line of inquiry.
"Horne died in 2001 after becoming ill as a result of a hunger strike. Serious offences have been committed and claimed in his name in the past."
The spokesman added: "A second company then contacted Thames Valley Police after they too received a suspect package in the mail.
"In this instance, the device wasn't activated and no-one was hurt. The two incidents are being linked."
Police are warning companies and individuals to be vigilant when dealing with incoming mail and to contact the police immediately about any suspcious items.
The construction of Oxford University's animal testing laboratory, in South Parks Road, has prompted a number of attacks on businesses and sports facilities by animal rights activists.
In November, activists claimed responsibility for a firebomb attack which caused £250,000 damage to lorries at Deans Foods egg farm, in Standlake.
Orchid Cellmark provides DNA testing services for food safety and selective trait breeding, and has laboratories in the United States and the UK.