Long before he was arrested in the shooting death of a woman at one of San Francisco's most popular tourist sites, Juan Francisco Lopez Sanchez was using the U.S.-Mexican border like a revolving door.
He was arrested while in the U.S. illegally and deported to his native Mexico five times from June 1994 to June 2009, only to slip back into the country. He served roughly 15 years in federal prison in three stints for illegal re-entry, completing the most recent stretch earlier this year.
But his habit of sneaking across the border is not all that uncommon. His case has illustrated yet again how border enforcement along the nearly 2,000-mile (3,220-kilometre) boundary with Mexico is an often frustrating task.
Last week, Sanchez was arrested and accused of killing 32-year-old Kathryn Steinle as she walked with her father. He pleaded not guilty Tuesday to murder charges.
It turned out that Sanchez, 45, was out on the streets because of San Francisco's "sanctuary" policy of minimal co-operation with federal immigration authorities.
"It's hard to physically prevent a committed immigrant from finding a way to get back in the U.S.," said Marc Rosenblum, deputy director for U.S. immigration policy at the Migration Policy Institute.
In 2013, a total of 18,498 people were sentenced for the federal crime of felony re-entry of the U.S. The offenders had been deported an average of 3.2 times each. The average sentence was 18 months, according to the U.S. Sentencing Commission.
A dramatic increase in border enforcement after the terror attacks of Sept. 11, 2001 made it increasingly difficult to cross. The Border Patrol doubled to more than 20,000 agents under President George W. Bush, and fences were erected on about one-third of the border.
Exactly how Sanchez managed to keep slipping back into the U.S. was not clear. But he was sent to federal prison in 1998, serving about 4 1/2 years, and again in 2003, where he put in nearly six years, and again in 2011, when he got close to four years.
After he completed that term, federal officials transferred him to San Francisco's jail in March to face a 20-year-old marijuana charge.
But local prosecutors dropped the drug charge, and the San Francisco sheriff, citing the city's sanctuary policy, released Sanchez in April, despite an Immigration and Customs Enforcement request to hold him for deportation.
ICE officials criticized the sheriff, who blamed the federal agency for not obtaining a warrant or court order that would have kept Sanchez locked up.
On Wednesday, the U.S. Bureau of Land Management said the gun used in the killing belonged to one of its rangers. Spokeswoman Dan Wilson said the service weapon had been stolen from the ranger's car in a break-in.
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Spagat reported from San Diego. Paul Elias in San Francisco and Amy Taxin in Los Angeles contributed to this report.
This content appears as provided to The Globe by the originating wire service. It has not been edited by Globe staff.