A secret U.S. government list of suspected terrorists has more than doubled in the past year.

Government figures provided to The Associate Press showed that the so-called no-fly list has skyrocketed from 10,000 suspected terrorists to around 21,000.

The U.S. bans all names on the no-fly list from flying to or within the country. Only about 500 of those names are said to be Americans.

After the failed bombing of Northwest Airlines Flight 253 on Christmas day in 2009, the government dramatically escalated the number of names being added to the list, according to AP.

Officials have refused to disclose exactly who is on the list or why a person might be placed on it.

The government also maintains a separate "terrorist watch list" that reportedly included over one million names by 2009.

Both lists have been prone to errors and misuse. In 2007, the Transportation and Security Administration (TSA) placed Walter F. Murphy, the McCormick Professor of Jurisprudence, Emeritus, at Princeton University, on the no-fly list after he criticized President George W. Bush. Indie actor Mark Ruffalo found himself on the terrorist watch list in 2010 for organizing a screening of an oil-drilling documentary. After criticizing the TSA in 2008, CNN's Drew Griffin also found himself on the list of potential terrorists.

It took an act of Congress to finally remove Nelson Mandela, the former South African president who helped end apartheid, from the list in 2008.

Photo: Flickr/Ben Popken