Washington: Santhosh Thomas, an Indian-American doctor, is at a loss to understand how his six-year-old daughter Alyssa turned up on the US government's terror watch list or how to get her off it.

The worst thing Alyssa has ever done is probably threatening her sister, and that, the doctor from Westlake, Ohio, says he's sure, is not enough to land her on the no-fly list of suspected terrorists.

"She may have threatened her sister, but I don't think that constitutes Homeland Security triggers," Thomas told CNN. An airline ticket agent informed the family of their predicament when they embarked on recent trip from Cleveland to Minneapolis.

"They said, ‘Well, she's on the list.' We're like, okay, what's the story? What do we have to do to get off the list? This isn't exactly the list we want to be on," Thomas was quoted as saying.

The Thomases were allowed to fly that day, but authorities told them to contact the US Department of Homeland Security to clear up the matter.

Now they've received a letter from the government addressed to six-year-old Alyssa, telling her that nothing in her file will be changed.

Federal authorities have acknowledged that such a no-fly list exists, but as a matter of national security, they won't comment on whose names are on it nor why.

"The watch lists are an important layer of security to prevent individuals with known or suspected ties to terrorism from flying," an unnamed spokesman for the Transportation Security Administration told Fox News.

"She's been flying since she was two months' old, so that has not been an issue," Alyssa's dad said.