SAN FRANCISCO — 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 within days, weeks or months. He served roughly 15 years in federal prison in three stints for illegal re-entry, completing his most recent stretch earlier this year.
But his habit of sneaking across the border over and over again is not all that uncommon. And probably no one outside law enforcement would have even paid much attention to Sanchez if not for what happened after he finished his latest stint behind bars.
Last week, he was arrested and accused of killing 32-year-old Kathryn Steinle as she strolled on a popular San Francisco pier with her father. It turned out that Sanchez, 45, was out on the streets because of San Francisco's "sanctuary" policy of minimal cooperation with federal immigration authorities.
The slaying has brought heavy criticism down on the city from politicians of both parties and become the latest flashpoint in the debate over how to deal with illegal immigration. On Wednesday, the San Francisco Sheriff's Department said it requested that Lopez be brought to them to face a 20-year-old marijuana possession charge and paid for his transportation before releasing him.
It illustrated yet again the way border enforcement along the nearly 2,000-mile boundary with Mexico is a gargantuan and often frustrating task.
"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. "There is no death penalty for immigration."
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.
Sanchez was deported the first time less than four months before President Bill Clinton launched Operation Gatekeeper to beef up border enforcement in San Diego, where the Border Patrol was badly overmatched by immigrants who typically stormed out of the hills by the dozens or the hundreds.
A dramatic increase in border enforcement from California to Texas after 9/11 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.
Still, the most determined and physically fit are able to cross.
He was sent to federal prison in 1998, serving about five years, and again in 2003, where he put in nearly six years, and again in 2011, when he served close to four years for being picked up by authorities trying to enter the country at a border crossing in Eagle Rock, Texas. He had given border inspectors a ruse that he was born in Arizona and therefore a U.S. citizen.
After he completed that term in March, federal officials transferred custody to the San Francisco Sheriff's Department to face the marijuana charge. Freya Horne, a sheriff's department lawyer, said agency policy is to request transfers of all prison inmates being released who have outstanding felony warrants.
But local prosecutors dropped the drug charge, and the San Francisco sheriff, citing the city's sanctuary policy and a 2013 city ordinance, released Sanchez in April, despite an Immigration and Customs Enforcement request to hold him for deportation.
ICE officials criticized the sheriff, who in turned blamed the federal agency for not obtaining a warrant or court order that would have kept Sanchez locked up.
After his arrest in the waterfront shooting, Sanchez told TV news stations he found the gun on the pier under a T-shirt and it accidentally went off.
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 while he was on business in San Francisco. The BLM employs some 200 armed rangers, who patrol public lands and enforce laws related to mining, grazing, timber and other activities.
Sanchez pleaded not guilty Tuesday to murder charges.
Spagat reported from San Diego. Paul Elias in San Francisco and Amy Taxin in Los Angeles contributed to this report.