Mexican police say they have arrested a suspect in the killing of a US border agent in Arizona in 2010.
Jesus Leonel Sanchez Meza, who police said used the alias Lionel Portillo-Meza, was detained in Sonora state.
He is one of five men accused of fatally shooting agent Brian Terry and is expected to be extradited to the US.
The killing of Mr Terry was at the centre of a scandal over a failed US operation to track guns smuggled across the US-Mexican border.
The agent was killed in a shootout will illegal immigrants with a weapon linked to Operation Fast and Furious, which saw US agents allowing hundreds of guns to enter Mexico illegally with the hope of tracking them to major arms dealers.
In the congressional investigation which followed the killing, two guns recovered from the scene were found to be among 2,000 weapons the authorities were supposed to be tracking.
Fast and Furious led to no arrests and many of the smuggled guns were later found at crime scenes, causing outrage in both the US and Mexico.
Mexico has repeatedly asked for US help in stemming the flow of weapons over the border.
In June, a US House of Representatives committee found Attorney General Eric Holder in contempt of Congress for withholding documents related to the operation.