According to an FBI fact sheet, 420,000 people were on the no-fly list in 2011.
A Gallup poll found that 67 percent of Americans believed more attacks were "very likely" or "somewhat likely."
The piece of mail was handed over to the FBI for further testing, CAIR said.
Syed Rizwan Farook may have contemplated an attack on a U.S. target as early as 2011.
Homegrown extremists such as the San Bernardino attackers give ISIS a terrifying new weapon.
Syed Farook and Tashfeen Malik were influenced by foreign militant organizations, FBI Director James Comey said.
Authorities say Syed Farook and Tashfeen Malik killed 14 people in California last week.
Gun manufacturers Smith & Wesson and Sturm Ruger & Co. see stocks rise about 7 percent.
Speech follows San Bernardino shooting that is being investigated as an act of terrorism.
Investigators look at whether Pakistani woman radicalized her American husband.
The FBI is investigating the deadly California shooting as an act of terrorism.
"I'm calling on Congress to close this loophole, now," the president said.
The California attack has opened up new debates and reopened old ones in the presidential election.
Members of the media entered Syed Farook and Tashfeen Malik's home on Friday.
The California attackers killed 14 people and wounded 21 others.
ISIS claims the attackers were followers of the militant group.
Even in a country where more people own more guns than anywhere else in the world, Brent Nicholson's cache is extraordinary
Here's how the @NRA Twitter account engages with a mass shooting: It doesn't.
Authorities identified the two suspects as U.S.-born Syed Farook, 28, and his wife, Tashfeen Malik, 27.
In most states, background checks are not required at gun shows and on the Internet.
Cruz says that the tragedy shows the U.S. is at war.
The shooting in Southern California killed 14 people and injured 17 on Wednesday.
The president said it is not yet clear what motivated the suspects.
Syed Farook and Tashfeen Malik were married and had a 6-month-old baby.