Catherine Banks, who served 22 years in the Marines, says she was on a Delta flight in San Francisco earlier this week, when she says a flight attendant walked up to her and said, "You need to get off the plane."

She says he told her the shirt she was wearing was "threatening." The t-shirt had a print that read, "Do Not Give in to the War Within ... End Veteran Suicide.”

Catherine told KNTV she was dumbfounded, and simply responded, "Are you kidding me?"

She says she explained she'd been in the service for over 2 decades, and was on her way to see her sister, who's also in the Marines -- but she claims the attendant said, "I don't care about your service" ... and told her she couldn't get back on the plane until she removed the shirt.

Banks says she was filled with anxiety as she stood on the jetway bridge, and -- after asking the flight attendant to turn around -- she took off the shirt and replaced it with a sweatshirt.

She was finally allowed to get back on the plane, but the flight had now been delayed and Catherine says she missed a connection.

As for what was so threatening about her shirt -- we've reached out to Delta, but no word back yet.

The data on military suicides are staggering -- reportedly 22 veterans die by suicide each day, and the rate across all branches of the armed forces is 32 percent.

We've reached out to Delta for comment ... so far, no word back.