Joan Rivers -- Melissa Rivers Has 'No Choice' But to Sue

Melissa Rivers will sue the medical center where Joan died and the doctors as well because it's the only way she can find out what really happened to her mom.

Sources connected to the Rivers family tell TMZ ... after the New York Dept. of Health and Human Services came out with its findings that Yorkville Endoscopy LLC was "deficient" in surgical services and medical staff ... Melissa tried to find out the root of the problems and if they caused her mom's death, but she was shut down.

We're told Melissa got advice from her lawyers ... the only way to know precisely what happened is to file a lawsuit, which entitles her to force doctors and facility staff to sit for sworn depositions and answer questions about what happened the day Joan stopped breathing.

Melissa, we're told, is frustrated because it's apparent the facility and the Dept. of Health know exactly what happened, so her only recourse is court.