London ? At least six people have been injured by seven letter bombs in Britain over the past three weeks, in what police said could be a campaign by animal rights extremists or a disgruntled motorist fed up with this country’s automobile laws – or both.

None of the injuries has been serious and the devices, contained in padded mailing envelopes, were designed to “cause shock and relatively minor injuries” using “pyrotechnic” material rather than “conventional explosives,” Anton Setchell of the Association of Chief Police Officers, who is coordinating the investigations, said at a news conference.

Setchell said police are keeping “a completely open mind” about who is responsible and possible links between the incidents, but that animal rights extremists and disgruntled motorists are “priority lines of investigation.”

Animal rights radicals have waged previous violent campaigns in Britain, including scores of firebombings. Another instance involved exhuming the mother-in-law of an owner of a family farm that raises guinea pigs for use in laboratory testing.

On Jan. 18, three commercial laboratories that do forensic work with police received letter bombs in the mail. One of them exploded, injuring one person. The other two were intercepted and did not explode, Setchell said.

The name of Barry Horne, an animal rights extremist and convicted firebomber who died on hunger strike in prison in 2001, was written on one of the packages. Police said they believe the three incidents are related, but they have not publicly suggested any link with this week’s bombings.

Other letter bombs exploded this week at three companies linked to the regulation of roads and drivers. On Monday, a woman was injured at the office of Capita Group Plc. in London, which has many government contracts, including design and management of the congestion control system that charges drivers the equivalent of about $16 a day to enter central London. The plan has been widely praised for reducing traffic but is unpopular among many drivers who believe the fee is excessive.

On Tuesday, two men suffered cuts and burns at the accounting firm Vantis Plc. in Wokingham, 40 miles southwest of London. The package that exploded was addressed to one of the firm’s clients, Speed Check Services, which provides highway speed-enforcement equipment and technology to police. A woman was injured Wednesday in a blast at the government’s Driver and Vehicle Licensing Agency main office in Swansea, Wales.