Criminals tend to obtain guns through multiple illegal pathways, including large-scale gun traffickers, who tend to provide criminals with relatively few guns. Federally licensed firearm dealers in the primary (new and used gun) market are regulated by the Bureau of Alcohol, Tobacco, Firearms, and Explosives (ATF). Firearm manufacturers are required to mark all firearms manufactured with serial numbers. This allows the ATF to trace guns involved in crimes back to their last Federal Firearms License (FFL) reported change of ownership transaction, although not past the first private sale involving any particular gun. A report by the ATF released in 1999 found that 0.4% of federally licensed dealers sold half of the guns used criminally in 1996 and 1997. This is sometimes done through "straw purchases." State laws, such as those in California, that restrict the number of gun purchases in a month may help stem such "straw purchases." States with gun registration and licensing laws are generally less likely to have guns initially sold there used in crimes. Similarly, crime guns tend to travel from states with weak gun laws to states with strict gun laws. An estimated 500,000 guns are stolen each year, becoming available to prohibited users. During the ATF's Youth Crime Gun Interdiction Initiative (YCGII), which involved expanded tracing of firearms recovered by law enforcement agencies, only 18% of guns used criminally that were recovered in 1998 were in possession of the original owner. Guns recovered by police during criminal investigations were often sold by legitimate retail sales outlets to legal owners, and then diverted to criminal use over relatively short times ranging from a few months to a few years, which makes them relatively new compared with firearms in general circulation.