The AP reports that the cause of Tim Russert's death is not yet known:

The cause of death was not immediately clear. The network said on its Web site that Russert died of a heart attack, but Michael A. Newman, Russert's internist, later said that resuscitation was begun immediately and continued at Sibley Memorial Hospital, to no avail. An autopsy was pending, Newman said.

Watch Tom Brokaw report:

Russert collapsed Friday afternoon in NBC's Washington bureau.

Russert was in the tracking booth, recording a track, when he collapsed. He returned from Italy Thursday night. His wife, Maureen Orth, and son Luke, are still in Italy.

TVNewser reminds us that "Around 1pmET this afternoon, Russert was conducting a political Q&A on MSNBC.com."

TMZ reports on the details of Tim Russert's death:

DC Fire tells us the call came in at 1:41 PM ET. We're told they transported one adult male patient in cardiac arrest to Sibley Hospital where he died.

Developing...

For a summary of Russert's life and career, click here.

Watch and read what media colleagues had to say in remembrance of Tim Russert.

Watch Tom Brokaw report the sad news of Tim Russert's death.

Watch Brian Williams react to the news of Russert's death.

Watch Andrea Mitchell cry remembering Tim Russert.

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