The size of the U.S. government's secret list of suspected terrorists who are banned from flying to or within the country has more than doubled in the past year.

The no-fly list jumped from about 10,000 known or suspected terrorists one year ago to about 21,000, according to government figures. About 500 are U.S. nationals.

The flood of new names began after the failed Christmas 2009 bombing of a Detroit, Michigan-bound jetliner when the U.S. government lowered the standard for putting people on the list and scoured its files for anyone who qualified. "We learned a lot about the watch-listing process and made strong improvements, which continue to this day," said Timothy Healy, director of the Terrorist Screening Center, which produces the no-fly list.

Among the most significant new standard is that a person doesn't have to be considered only a threat to aviation to be placed on the list.

People considered a broader threat to domestic or international security or who attended a terror training camp are also included, said a U.S. counter-terrorism official who spoke on condition of anonymity. As agencies complete the reviews of their files, the pace of growth is expected to slow, said the counter-terrorism official.