A letter bomb exploded Tuesday at an office southwest of London, injuring two people, in the second attack of its kind in two days, officials said.

Army bomb-disposal units surrounded an office belonging to Vantis PLC, a tax and accounting company, after the small explosion at 9 a.m. in Wokingham, 40 miles southwest of London.

Thames Valley police said it was "likely" that a piece of mail had ignited and caused the "small explosion."

It was too soon to say whether the explosion was linked to a letter bomb that detonated Monday in central London, injuring one person, police added.

On Monday, a padded envelope exploded at an office belonging to Capita Group PLC, which administers an $16 daily fee for vehicles meant to cut down on traffic in central London and collects television licensing fees.

A Vantis spokesman, speaking on condition of anonymity in line with company practice, confirmed the attack.

The two employees injured in the explosion were not hospitalized, he said, adding: "My understanding is that they are in a bit of shock."

The Irish Republican Army used letter bombs in the early 1970s as part of its campaign to end British rule in Northern Ireland, targeting numerous British government offices. The tactic maimed dozens of people but killed nobody.

In recent years, animal rights activists have also sent letter bombs to people involved in animal testing.

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