We have not received as many tips or leads as we would like

FBI Director Robert Mueller

Mr Mueller said the agency was aggressively investigating the confirmed cases of anthrax, but admitted it was no closer to finding the culprit.

"We have not received as many tips or leads as we would like," he said.

Seventeenth case

Later on Friday, a 17th case of anthrax in the United States was confirmed.

The victim is an employee at the New York Post, who has contracted the less serious skin variety of the disease.

Mr Mueller said the FBI was pursuing more than 1,000 leads, including more than 100 overseas, and had carried out more than 2,000 interviews.

"Despite speculation about the possible source of the anthrax and the motive for the attacks, nothing yet has been ruled out," he said.

"And so today I am calling upon all of you in the news organisations to basically speak for us, to ask our fellow citizens to join us in trying to bring leads to the front that will help us solve both the anthrax investigation, but also the 11 September hijacking investigations."

Anthrax spreads

17 confirmed infections 4 deaths (1 in Florida, 2 in Washington DC, 1 in New York) 6 ill with inhalation anthrax 7 cases skin anthrax 13,300 postal workers taking antibiotics as protective measure

Asked whether the FBI was any closer to knowing the source of the anthrax or who had mailed it, he avoided answering the question directly, says the BBC's Washington correspondent.

In a separate development, the German authorities have said two packages suspected of containing anthrax tested negative.

Earlier, a Pakistani newspaper, the Daily Jang, said it had received a letter containing white powder which had tested positive for anthrax.

On Wednesday, a hospital worker from New York died from inhalation anthrax, the fourth fatal victim of the disease in October.