BELFAST, Northern Ireland (AP) _ A prison officer accused of mistreating Irish Republican Army guerrillas was killed Tuesday by a bomb that destroyed his car as he was driving through Belfast.

Hours later, an estimated 500 pounds of explosives inside a car blew up outside a building in Belfast used by the Inland Revenue tax service, starting a fire and causing extensive damage but no injuries, according to police.

The IRA asserted responsibility for both bombings.

Police said Brian Armour, 48, was killed when a bomb exploded in his car as he drove near his home in mainly Protestant east Belfast. An IRA statement said it was the second attempt to kill the officer.

Officials said Armour was vice chairman of the Prison Officers Association, a professional group, and worked at the Maze prison outside Belfast where dozens of IRA inmates are held.

In a statement to the local news media, the IRA claimed Armour had ''a central role in organizing and carrying out the systematic torture and brutalization of hundreds of Republican prisoners'' during a period of Maze prison protests that included the deaths of 10 inmates in hunger strikes in 1981.

It also said Armour had recently advocated harsh treatment of IRA inmates at the Maze and at Crumlin Road jail in Belfast.

The statement said IRA members planted a 2-pound charge of plastic explosives in his car but it failed to explode. A second packet of 1 1/2 pounds of explosives then was placed in the car and it exploded, according to the statement.

A police spokesman said a telephoned warning gave security units 30 minutes to evacuate the area around the Inland Revenue building. No casualties were reported, but the spokesman said an elderly woman was treated for shock.

Armour was the 81st person to be killed this year in Northern Ireland's sectarian violence. The known death toll since the IRA began its violent campaign in 1969 is 2,699.

The IRA is fighting to unite Northern Ireland, a British province where Protestants outnumber Roman Catholics 3-2, with the predominantly Catholic Republic of Ireland.

It was announced Tuesday that Sir John Hermon will retire in May after a decade as chief constable of the Royal Ulster Constabulary, the province's police force. No successor has been named.

During his tenure the 13,000-member constabulary supplanted British army units as the province's main security force.

British officials have said Hermon, 59, molded an effective anti-terrorist force, but 85 percent of the constabulary's members are Protestants and efforts to win support from Catholic leaders have failed.