VIENNA, Austria (AP) _ A letter bomb addressed to a popular writer of metaphysical books exploded Monday as a police officer was trying to defuse it, injuring him slightly.

The bomb was sent to Lotte Ingrisch, the widow of Gottfried von Einem, Austria's best-known contemporary composer. She is also the stepmother of Interior Minister Caspar Einem.

The police officer suffered bruises on his hands as he tried to disengage the letter bomb, the first since December 1995, but was otherwise uninjured.

On five occasions since December 1993 multiple letter bombs have exploded on the same day, police said. A total of nine people have been injured, some seriously.

One or more right-wing extremists who have attacked minorities and their supporters in recent years are suspected of sending the bombs.

The letter bomb was mailed to Ingrisch's former address in the Lower Austria province, but the post office returned it to the sender because Ingrisch had moved.

The return address on the bomb was ``Albine Schneider, Institute for Astrology'' in Vienna. When people at the institute received it, they became suspicious and alerted police.

The letter was mailed on Dec. 2.

The first of string of explosions occurred Dec. 3, 1993, when Helmut Zilk, then-mayor of Vienna, lost part of his left hand. On Dec. 11, 1995, four letter bombs exploded. No one was injured.

Ingrisch said she had received strange letters before, mostly with a nationalist tone, but said on state television, ``I am not afraid.''