1:44 PM PT -- Sources familiar with the situation tell TMZ Shandling died from a massive heart attack, with no prior warning whatsoever.

Comedian Garry Shandling died at an L.A. area hospital on Thursday ... TMZ has learned.

The 66-year-old star was not suffering from any illness ... as far as we know ... so, it appears this was sudden. A source connected to Shandling says he was healthy and speaking to people on Thursday morning.

Our source says there was a 911 call from "The Larry Sanders Show" star's home, and he was transported to the hospital. We're told Shandling was alive when he arrived at the hospital.

Last weekend, Shandling was hanging with friends Kathy Griffin and Bob Odenkirk ... who also co-starred with Garry on 'Larry Sanders.'

Story developing ...