You've heard this shocking "fact" before -- on TV and radio, in newspapers, on the Internet and from the highest politicians in the land: 90 percent of the weapons used to commit crimes in Mexico come from the United States.

-- Secretary of State Hillary Clinton said it to reporters on a flight to Mexico City.

-- CBS newsman Bob Schieffer referred to it while interviewing President Obama.

-- California Sen. Dianne Feinstein said at a Senate hearing: "It is unacceptable to have 90 percent of the guns that are picked up in Mexico and used to shoot judges, police officers and mayors ... come from the United States."

-- William Hoover, assistant director for field operations at the Bureau of Alcohol, Tobacco, Firearms and Explosives, testified in the House of Representatives that "there is more than enough evidence to indicate that over 90 percent of the firearms that have either been recovered in, or interdicted in transport to Mexico, originated from various sources within the United States."

There's just one problem with the 90 percent "statistic" and it's a big one:

It's just not true.

In fact, it's not even close. The fact is, only 17 percent of guns found at Mexican crime scenes have been traced to the U.S.