A letter bomb sent to a Birmingham firm was designed to shock rather than kill, police have said.
Officers have linked a device sent in an A5 envelope to Forensic Science Service in Chelmsley Wood with similar ones sent to two Oxfordshire firms.
Police said they are pursuing "priority lines of inquiry" and that animal rights extremists or a disgruntled motorist could be behind the devices.
The Chelmsley Wood company is a forensics laboratory, as is one of the Oxfordshire sites, while the other is involved in DNA testing.
The name of animal right activist Barry Horne, who died in prison in 2001 following a hunger strike, had been scrawled on the back of one envelope sent to a firm in Abingdon.
Thames Valley Police said they have investigated any link between Barry Horne and the firms targeted.
But a spokesman could not confirm reports that all three firms were involved in the prosecution of Mr Horne.
The Forensic Science Service, which received a "suspect package" on January 19, is a Government-owned company, supplies forensic sciences services to police forces in England and Wales, dealing with hundreds of thousands of cases every year, with experts attending crime scenes and appearing as witnesses in court.
The investigation has widened came after letter bombs were also sent to firms in London, Wokingham and the DVLA in Swansea.