Story highlights Only about 10,000 people were on the list a year ago

New criteria are being used to determine who goes on the list, an official says

Those "deemed to be a threat to national security" can be banned, the official says

The U.S. government's list of suspected terrorists who are banned from flying to the United States or within its borders has more than doubled over the past year, a counterterrorism official told CNN Thursday.

The "no fly" list produced by the FBI now has approximately 21,000 names on it, according to the official, who has knowledge of the government's figures. One year ago about 10,000 individuals were on it.

Only about 500 people currently on the no-fly list are Americans, the official said.

The dramatic jump in the numbers resulted from reforms made after a Nigerian man with explosives in his underwear was able to get on an international flight bound for Detroit on Christmas Day 2009. It was later learned the father of Umar Farouk AbdulMutallab had gone to the U.S. Embassy in Nigeria prior to Christmas to raise concern about his son, but that did not result in his going on the no-fly roster.

The United States can now ban people from flying who are "deemed to be a threat to national security" or who had gone to terrorist training camps, said the official. The earlier standard was to block only those considered a specific threat to try to bring down a plane.

Another reform would have been a particular help in the case of AbdulMutallab. Analysts can now use single-source information, if it's considered credible, to recommend someone for one of the government's terror watch lists, including the no-fly list.

The official said AbdulMutallab's father was a respected businessman and would have been viewed as a credible single source under the new procedures. The official said all the changes are evidence the listing process has "matured."

The government also has a much larger list, called the Terrorist Screening Database, with approximately 510,000 names currently on it. The smaller no-fly list is a subset of that.

About 1,000 changes are made to the catalog of possible terrorists each day. Names are added and deleted, or more information is included on individuals.

AbdulMutallab pleaded guilty in October to charges that carry a mandatory sentence of life in prison. In a courtroom statement, he said he had traveled to Yemen and was "greatly inspired" by U.S.-born cleric Anwar al-Awlaki to participate in the bombing plot.

AbdulMutallab is scheduled for sentencing on February 16. Al-Awlaki was killed in Yemen last year by a U.S. drone strike.