By Marty Graham

SAN DIEGO (Reuters) - A Tijuana architect who admitted smuggling drugs across the U.S.-Mexico border was ordered on Monday to serve six months in prison in an unusually lenient sentence taking into account that he committed the crime under threats to his family.

Eugenio Velazquez, who designed one of Tijuana's most widely recognized landmarks - the cube-shaped addition to the Tijuana Cultural Center - also was ordered to remain under home confinement for six months following his release from federal prison.

Velazquez had pleaded guilty to a single count of illegally importing narcotics under a deal with prosecutors that spared him what would have been a much harsher penalty.

Prosecutors said they agreed to recommend the lighter-than-usual sentence because Velazquez was able to corroborate assertions that he had been coerced into trying to sneak cocaine into California out of fear for the safety of his family.

Claims of coercion in similar cases frequently go unproven, they said.

"People we arrest have been saying it all along," said Joe Garcia, the deputy special agent in charge of the San Diego office of Immigrations and Customs Enforcement. "More times than not, we are able to debunk those claims."

For Velazquez, who also designed the new Tijuana police station and is currently designing a new Roman Catholic cathedral in Baja, Mexico, his ordeal began with a coin toss, according to court documents.

A new client had offered Velazquez and an unnamed physician protection for their families, then told them at gunpoint they owed $40,000 for that protection or could pay it off by smuggling drugs.

Velazquez and the doctor flipped a coin to decide who would smuggle the drugs and Velazquez lost. He was arrested at the border in March with 13 lbs (5.9 kg) of cocaine in his car.

"The way the duress laws are written, it's almost impossible to build a defense," the architect's lawyer, Jeremy Warren, told Reuters. "We were fortunate that Mr. Velazquez is such an upright citizen and devout Catholic - and that the doctor was willing to give a statement."

The sentencing of Velazquez came a month after a talented Mexican opera singer, Maximino Melchor Vazquez, 23, received a nine-year prison term for smuggling methamphetamine across the border. His lawyers likewise argued, unsuccessfully, that Vazquez engaged in smuggling out of fear for his life.

Victor Clark Alfaro, an activist with the Binational Center for Human Rights, says that with tightened border security, smugglers in Mexico have increasingly sought to use coercion and threats to move drugs into the United States in the possession of otherwise law-abiding citizens.

"The drug organizations say they will harm the person's family - they show you pictures of your family they took - and with the violence in this country, you believe they will kill you or your family," he said, adding that there had also been increasing instances of unsuspecting motorists have drugs planted in their vehicles by smugglers.

(Editing by Cynthia Johnston, Steve Gorman and Lisa Shumaker)