Friends and colleagues have been paying tribute to Dr David Kelly.
Dr Kelly wanted to help journalists understand a complex topic
Scott Ritter, who worked with him as part of the UN weapons inspections team in Iraq, said he was a man of "integrity, character, and somebody who cared deeply about his country".
Mr Ritter, who lead the UN inspection team up until it left Iraq in 1998, told BBC Radio 4's Today programme that Dr Kelly was somebody who had doggedly pursued the Iraqi biological weapons program and who had never caved in to pressure.
He said: "My experience of David is that he is a man who does his job and does it quietly.
"While a gentle man, he had a core of steel in him. I've seen him interact with Iraq government officials; there was no give in this man."
Tom Mangold, a television journalist and close friend of Dr Kelly, said he had made himself available to many serious journalists because he wanted to help them understand a complex topic.
Mr Mangold said: "He was a man whose brain could boil water, he used words with tremendous precision, he used them as weapons.
"There was nothing he didn't know about biological warfare and there wasn't much he didn't know about WMD."
Mr Mangold said Dr Kelly had not been particularly interested in journalism or journalists.
"He was passionately interested in what happens in Iraq," he said.
"That was one of the reasons why Saddam Hussein wanted him out more than anybody else."
Richard Butler, former chief United Nation weapons inspector in
Iraq, said he had known David Kelly well.
"He was one of the lead biologists on my team. I liked him as a man. Above all I had a deep faith in his honesty and professional integrity. He's a great loss.
"I will never forget the way he always insisted to me no
matter what political pressure that was put on us, that we had to tell the
truth, absolutely the truth, maybe this is part of why these terrible events
have taken place."