EXCLUSIVE: 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.

What's true, an ATF spokeswoman told FOXNews.com, in a clarification of the  statistic used by her own agency's assistant director, "is that over 90 percent  of the traced firearms originate from the U.S."

But a large percentage of the guns recovered in Mexico do not get sent back  to the U.S. for tracing, because it is obvious from their markings that they do  not come from the U.S.

"Not every weapon seized in Mexico has a serial number on it that would make  it traceable, and the U.S. effort to trace weapons really only extends to  weapons that have been in the U.S. market," Matt Allen, special agent of U.S.  Immigration and Customs Enforcement (ICE), told FOX News.

Video:Click here to watch more.

A Look at the Numbers

In 2007-2008, according to ATF Special Agent William Newell, Mexico submitted  11,000 guns to the ATF for tracing. Close to 6,000 were successfully traced --  and of those, 90 percent -- 5,114 to be exact, according to testimony in  Congress by William Hoover -- were found to have come from the U.S.

But in those same two years, according to the Mexican government, 29,000 guns  were recovered at crime scenes.

In other words, 68 percent of the guns that were recovered were never  submitted for tracing. And when you weed out the roughly 6,000 guns that could  not be traced from the remaining 32 percent, it means 83 percent of the guns  found at crime scenes in Mexico could not be traced to the U.S.

So, if not from the U.S., where do they come from? There are a variety of  sources:

-- The Black Market. Mexico is a virtual arms bazaar, with fragmentation  grenades from South  Korea, AK-47s from China,  and shoulder-fired rocket launchers from Spain, Israel and former  Soviet bloc manufacturers.

-- Russian crime organizations. Interpol  says Russian Mafia groups such as Poldolskaya and Moscow-based Solntsevskaya are  actively trafficking drugs and arms in Mexico.

- South America. During the late 1990s, the Revolutionary Armed Forces of Colombia (FARC)  established a clandestine arms smuggling and drug trafficking partnership with  the Tijuana cartel, according to the Federal Research Division report from the Library  of Congress.

-- Asia. According to a 2006 Amnesty International Report, China has provided  arms to countries in Asia, Africa  and Latin  America. Chinese assault weapons and Korean explosives have been recovered  in Mexico.

-- The Mexican Army. More than 150,000 soldiers deserted in the last six  years, according to Mexican Congressman Robert Badillo. Many took their weapons  with them, including the standard issue M-16 assault rifle made in Belgium.

-- Guatemala.  U.S. intelligence agencies say traffickers move immigrants, stolen cars, guns  and drugs, including most of America's cocaine, along the porous  Mexican-Guatemalan border. On March 27, La Hora, a Guatemalan newspaper,  reported that police seized 500 grenades and a load of AK-47s on the border.  Police say the cache was transported by a Mexican drug cartel operating out of  Ixcan, a border town.

'These Don't Come From El Paso'

Ed Head, a firearms instructor in Arizona who spent 24 years with the U.S.  Border Patrol, recently displayed an array of weapons considered "assault  rifles" that are similar to those recovered in Mexico, but are unavailable for  sale in the U.S.

"These kinds of guns -- the auto versions of these guns -- they are not  coming from El Paso," he said. "They are coming from other sources. They are  brought in from Guatemala. They are brought in from places like China. They are  being diverted from the military. But you don't get these guns from the  U.S."

Some guns, he said, "are legitimately shipped to the government of Mexico, by  Colt, for example, in the United States. They are approved by the U.S.  government for use by the Mexican military service. The guns end up in Mexico  that way -- the fully auto versions -- they are not smuggled in across the  river."

Many of the fully automatic weapons that have been seized in Mexico cannot be  found in the U.S., but they are not uncommon in the Third World.

The Mexican government said it has seized 2,239 grenades in the last two  years -- but those grenades and the rocket-propelled grenades (RPGs) are  unavailable in U.S. gun shops. The ones used in an attack on the U.S. Consulate  in Monterrey  in October and a TV station in January were made in South Korea. Almost 70  similar grenades were seized in February in the bottom of a truck entering  Mexico from Guatemala.

"Most of these weapons are being smuggled from Central American countries or  by sea, eluding U.S. and Mexican monitors who are focused on the smuggling of  semi-automatic and conventional weapons purchased from dealers in the U.S.  border states of Texas, New Mexico, Arizona and California," according to a  report in the Los Angeles Times.

Boatloads of Weapons

So why would the Mexican drug cartels, which last year grossed between $17  billion and $38 billion, bother buying single-shot rifles, and force thousands  of unknown "straw" buyers in the U.S. through a government background check,  when they can buy boatloads of fully automatic M-16s and assault rifles from  China, Israel or South  Africa?

Alberto Islas, a security consultant who advises the Mexican government, says  the drug cartels are using the Guatemalan border to move black market weapons.  Some are left over from the Central American wars the United States helped  fight; others, like the grenades and launchers, are South Korean, Israeli and  Spanish. Some were legally supplied to the Mexican government; others were sold  by corrupt military officers or officials.

The exaggeration of United States "responsibility" for the lawlessness in  Mexico extends even beyond the "90-percent" falsehood -- and some Second  Amendment activists believe it's designed to promote more restrictive  gun-control laws in the U.S.

In a remarkable claim, Auturo Sarukhan, the Mexican ambassador to the U.S.,  said Mexico seizes 2,000 guns a day from the United States -- 730,000 a year.  That's a far cry from the official statistic from the Mexican attorney general's  office, which says Mexico seized 29,000 weapons in all of 2007 and 2008.

Chris Cox, spokesman for the National  Rifle Association, blames the media and anti-gun politicians in the U.S. for  misrepresenting where Mexican weapons come from.

"Reporter after politician after news anchor just disregards the truth on  this," Cox said. "The numbers are intentionally used to weaken the Second  Amendment."

"The predominant source of guns in Mexico is Central and South America. You  also have Russian, Chinese and Israeli guns. It's estimated that over 100,000  soldiers deserted the army to work for the drug cartels, and that ignores all  the police. How many of them took their weapons with  them?"