Rivers and her daughter Melissa had also for many years led the coverage of many of the network's red carpet events, from the Golden Globes to the Academy Awards.
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"She was a colleague and friend, as we worked together on E!, and she was also a treasured guest on my radio show many times," Ryan Seacrest said about Rivers. "Regardless of the forum, Joan was full of funny, witty surprises and she had so much spirit."
The network has yet to comment on the future of "Fashion Police" or any other programming involving Rivers.
"Right now we are mourning our beloved Joan," E! said in a statement. Regardless of what the network does, it'll be incredibly difficult for E! to fill the shoes of the iconic star.