Tim Russert, moderator of NBC's "Meet the Press," author and a respected political journalist, died Friday of unknown circumstances, first thought to be a heart attack.

NBC's D.C. bureau chief was born in 1950, and was only 58.

NBC announced the news during the mid-afternoon programming as Tom Brokaw, the former anchor of "NBC Nightly News" and a longtime friend of Russert, announced that he had collapsed while on the job for "Meet the Press".

Described as a "journalist's journalist" by Wolf Blitzer for CNN, networks Fox and Bloomberg Television - in addition to NBC's cable ops, MSNBC and CNBC - launched extensive coverage focusing on shocked peer and political reactions.

"We were stunned and deeply saddened to hear the news." said former president Bill Clinton and his wife Senator Hillary in a statement. "He helped informed the American people to make the democracy stronger..."

Republican presidential contender Sen. John McCain called him "the preeminent political journalist of his generation."

Bob Schieffer, host of CBS' Sunday talker "Face the Nation," said: "Tim was the best of our profession. He asked the best questions and then he listened for the answer. He delighted in scooping me and I felt the same way when I scooped him. When you slipped one past ol' Russert, you felt as though you had hit a home run off the best pitcher in the league."

President George Bush expressed sorrow. "Laura and I are deeply saddened by the sudden passing of Tim Russert," Bush said in a statement. "Those of us who knew and worked with Tim, his many friends, and the millions of Americans who loyally followed his career on the air will all miss him... He was an institution in both news and politics for more than two decades. Tim was a tough and hardworking newsman. He was always well-informed and thorough in his interviews. And he was as gregarious off the set as he was prepared on it."

Brokaw had said that Russert had just recently returned from a family trip to Italy.

A devout Catholic, Russert was also author of two best-selling books, and Time magazine named him this year as one of the most 100 influential people in the world.