An operation headed by the Bureau of
Alcohol, Tobacco and Firearms, started in 2009. The ATF thought rather than arresting gun smugglers, they'd let the guns roam free
like cattle, allowing smuggling to continue, gambling it would lead them to cartel bosses. Instead they lost track of over 2,000 guns which has been been linked to numerous murders in the US and Mexico.
This
failure of an operation ironically later led to the deaths of US Border Patrol and and ATF agents.