NEW YORK (AP) --

Tim Russert,

who pointedly but politely questioned hundreds of the powerful and

influential as moderator of NBC's "Meet the Press,"

died suddenly Friday while preparing for his weekly broadcast.

The network's Washington bureau chief was 58.

In addition to his weekly program,

Russert appeared on the network's other news shows,

was moderator for numerous political debates and wrote two best-selling books.

President Bush, informed of Russert's death while at dinner in Paris,

swiftly issued a statement of condolence that praised the NBC newsman as

"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."

NBC interrupted its regular programming with news of Russert's death,

and in the ensuing moments,

familiar faces such as Tom Brokaw,

Andrea Mitchell and Brian Williams took turns mourning his loss.

Williams called him "aggressively unfancy."

Our hearts are broken," said Mitchell,

who appeared emotional at times as she recalled her long-time colleague.

Bob Schieffer, Russert's competitor on CBS'

"Face the Nation," said the two men delighted in scooping each other.

"When you slipped one past 'ol Russert," he said.

 "You felt as though you had hit a home run off the best pitcher in the league.

 I just loved Tim and I will miss him more than I can say."

The cause of death was not immediately clear.

The network said on its Web site that Russert died of a heart attack,

but spokeswoman Jenny Tartikoff later said,

"We do not know the cause yet."

Russert collapsed while preparing for his show at NBC's studios

across town from the White House.

Russert, of Buffalo, N.Y.,

 took the helm of the Sunday news show in December 1991

and turned it into the nation's most widely watched program of its type.

His signature trait there was an unrelenting style of questioning

that made some politicians reluctant to appear,

yet confident that they could claim extra credibility

if they survived his grilling intact.

He was also a senior vice president at NBC, and this year,

Time Magazine named him one of the 100 most influential people

in the world.

Russert had Buffalo's blue collar roots,

a Jesuit education,

a law degree and

a Democratic pedigree that came from his turn as an aide

to the late Sen. Daniel Patrick Moynihan of New York.

One of his books, "Big Russ and Me,"

was about his relationship with his father.

On Sunday's program,

Russert was to have interviewed Sens. Lindsey Graham, R-S.C.,

 a national co-chair of the McCain campaign, and Joe Biden, D-Del.,

an Obama supporter,

in a debate format as surrogates for the

two presidential candidates.

Praise flowed quickly from those who knew Russert

across the television interview room.

Senate Majority Leader Harry Reid said Russert

 was "the best in the business at keeping his interview subjects honest."

"There wasn't a better interviewer in television,"

Sen. Barack Obama, the Democratic presidential contender,

told reporters in Ohio.

Republican Sen. John McCain of Arizona,

Obama's rival for the White House,

hailed Russert as the "pre-eminent journalist of his generation."

Rep. John Boehner of Ohio, the House Republican leader,

said Russert was "one of the smartest,

 toughest television news journalists of all time ....

I can say from experience that joining Tim on Meet The Press

was one of the greatest tests any public official could face."

Carl P. Leubsdorf, president of the Gridiron Club,

an organization of journalists, said in a statement,

 "It was a measure of the degree to which Tim Russert was respected

in the journalistic world that he was the first broadcaster

elected to membership in the Gridiron Club

after the rules were changed in 2004

to end our century-old restriction to print journalists."

"He was an enthusiastic member and a willing participant in our shows.

His fellow Gridiron members join with all of those

who knew and respected Tim in mourning his untimely death."

"It is my sad duty to report this afternoon"

that Russert collapsed and died while working

 in the network's Washington studios,

Brokaw said when he came on the air.

"He'll be missed as he was loved — greatly,"

Brokaw said.

The network said on its Web Site that Russert

had been recording voiceovers for this Sunday's

"Meet The Press" when he was stricken.

Russert had dozens of honorary college degrees,

and numerous professional awards.

He won an Emmy for his role

 in the coverage of President Ronald reagan's funeral

in 2004.

He was married to Maureen Orth,

a writer for Vanity Fair Magazine.

 The couple had one son, Luke.