Salt-N-Pepa's Sandra Denton Demands Southwest Investigate Plane Incident

Salt-N-Pepa's Sandra Denton is putting the heat on Southwest after being kicked off a flight last month ... demanding the airline investigate the issue and preserve any evidence in case this turns into a lawsuit.

In a legal letter obtained by TMZ ... Pepa's lawyers call on Southwest to get the ball rolling on investigating her removal from a July flight ... echoing her previous sentiments that a male agent engaged in "inappropriate and unnecessary action" in handling the situation.

The attorneys are going after Southwest with a number of allegations ... including their belief Sandra was racially profiled and discriminated against during the encounter. They've also called into question how certain policies were enforced.

In the letter, Pep's attorneys say she's suffered from embarrassment, harassment, and loss of income over the ordeal ... and so, they want Southwest to look into everything that happened -- and provide "fair and transparent resolution" of the matter.

They also put the airline on notice that it should preserve all evidence and prevent the destruction and/or loss of any info tied to the matter.

Translation: We might sue ya, Southwest, so keep all potential evidence handy for a trial.

Her legal team is giving Southwest until the end of the month to respond to the letter.

TMZ broke the story ... the rapper told us she was booted off her flight in Las Vegas, despite having purchased 2 tickets for medical reasons. Pepa said she was pressured to give up her additional seat, since she wasn't in a cast ... even though, she says, she had a knee brace on at the time.

She claims she was once again asked to give up her additional seat when a man boarded the fully booked plane and was unable to find a seat.