CANBERRA Australia AP A letter bomb exploded early Wednesday morning at Canberra's regional mail center in the industrial suburb of Fyshwick injuring one postal worker. Police said the man's injuries are not serious but they have closed the center fearing that more devices might be present. The ceiling of the building was damaged The explosion occurred in the early hours of Wednesday local time. Police say their inquiries might take a week. ``It was a well-constructed sophisticated letter bomb' Police Sgt. Ron Garbutt told Australian Broadcasting Corp. radio. ``To me that appears as though the person constructing it knew what they were doing and was certainly not an amateurish attempt at making a letter bomb.'' The police have no idea who the bomb was addressed to or where it came from Garbutt said. at-pjs APW19981201.1051.txt.body.html APW19981201.0167.txt.body.html