The FBI says one man in the United States likely wrote three anthrax-tainted letters sent to media and government offices, calling him an "opportunist" who took advantage of the September 11th attacks.
FBI officials stressed they had not ruled out that the person who sent the letters may belong to a foreign network like al Qaeda, the group run by Saudi-born dissident Osama bin Laden, who is blamed for the September 11 suicide hijacked plane attacks on New York and Washington. Thousands were killed.
"We're not ruling anybody out but we're certainly looking in that direction now, in terms of someone being domestic," one FBI official said when asked if the suspect was US-based.
"He is likely a person living in the US who took advantage of September 11th. He is an opportunist and he took advantage," the official said, referring to the confusion and uncertainty that followed the September attacks.
"At this point and time there is no direct or clear linkage between this particular series of incidents and any terrorist cell or network," he said.
Later, US Attorney General Mr John Ashcroft said in a transcript of an interview to air on Friday night on CNN's Larry King Livethat authorities did not have "a level of confidence that makes certain the fact that these are domestic."
FBI officials painted a picture, based on behavioural and linguistic studies of the letters, of the man they say was responsible for sending three anthrax-tainted letters - one to US Senate Majority Leader Mr Tom Daschle and two identical letters to the New York Postand NBC anchor Mr Tom Brokaw.
Officials said the suspect is an adult male who might work in a laboratory who probably has a scientific background and is comfortable working with hazardous material like anthrax.
He was suspected of working alone, and was seen as a "loner" who spent most of his time by himself, the FBI said. He probably took antibiotics or was vaccinated against anthrax to ensure he did not contract the disease, officials added.