The U.S. FBI and DEA (Drug Enforcement Agency) uncovered a plot by two Pakistanis and one a Pakistani born U.S. citizen to sell a ton of heroin and five tons of hashish in southern California, and use the money to buy weapons (including Stinger portable anti-aircraft weapons) for al Qaeda. The majority of the heroin produced on the planet now comes from Afghanistan and Pakistani gangs control much of the trade. While the gangs are most concerned with making lots of money, weapons, particularly exotic weaponry, are valuable commodities in the tribal areas of Pakistan where the drug trade thrives. So trading guns for drugs makes sense to these people.
The FBI has long had success in running sting operations for people looking to buy American weapons illegally. For this reason, most terrorist and criminal groups prefer to deal with Eastern European gun runners, who are rarely infiltrated by the FBI or CIA. But the East European arms dealers only provide Cold War surplus Russian weapons. Russian portable anti-aircraft missiles are not as good as Stingers, but can get the job done, particularly against smaller civilian aircraft. But the Stinger is considered something of a wonder weapon by terrorists who are familiar with that missiles use during the 1980s war with Russia in Afghanistan. The three drug dealers were arrested in Hong Kong, which has an extradition treaty with the United States.