Ever since taking office last year as the nation's first Hispanic female governor, New Mexico's Susana Martinez found her family tree scrutinized over whether her Mexican-born paternal grandfather was an illegal immigrant.

Documents obtained by The Associated Press, however, show that he was lawfully admitted to the U.S. as a permanent resident in 1918 and became a U.S. citizen in 1942, something not even Martinez knew and a discovery that removes a potential trouble spot for someone mentioned as a possible vice presidential running mate for Mitt Romney.

Martinez was surprised at the news, but maintained that his status, citizen or not, didn't affect her political views. "I embrace lawful immigration," she said. "I think it's what makes America wonderful."

The first-term governor insists she's not interested in and wouldn't accept a spot on the GOP ticket. But resolving the questions lifts a "hot potato off her plate," said Larry Sabato, director of the University of Virginia's Center for Politics.

"It's been a controversy and it's always mentioned when her name comes up in connection with the vice presidency," he said. "Does it help her? Sure, if Romney has any interest in her and if she has any interest in accepting, if offered."

The questions arose after the former prosecutor advocated early last year the repeal of a 2003 law that allowed foreign nationals without Social Security numbers, including illegal immigrants, to get driver's licenses.

News accounts about a 1930 census initially fueled the idea that Martinez's paternal grandparents had illegally entered the country. The census used an "AL" to designate that her grandparents were "aliens."

That designation wasn't an indication of whether they lawfully entered the U.S. It only meant they were not citizens and hadn't filed papers declaring their intent to become one, according to historians and immigration experts.

Critics who opposed Martinez's proposal seized on the reports, arguing that her family offered an example of illegal immigrants coming to the U.S. for a better life and that a repeal of the law would deny others the same chance.

When the questions arose, she couldn't turn to her parents. Her father has Alzheimer's disease and her mother died in 2006. The grandfather died in 1976.

So she initially accepted media accounts and acknowledged that it appeared her grandparents had come to the U.S. without immigration documents. Meanwhile, her proposal died in the Democratic-controlled Legislature.

To try to deal with lingering questions, her political organization last fall found documents that indicated her grandfather, Adolfo R. Martinez, had crossed the border several times in the early 1900s.

The immigration documents showed her paternal grandparents followed common practices in crossing what was essentially an open border at the time. The documents weren't clear that he had been lawfully admitted for permanent residency.

The AP obtained a "certificate of naturalization," dated April 6, 1942, from the National Archives Southwest Region center in Fort Worth, Texas. When shown the document, Martinez said she was unaware that her grandfather had become a citizen.

Martinez said the citizenship information appears to resolve the immigration questions about her grandfather, but wasn't relevant to her political future or her continuing efforts to stop driver's licenses for illegal immigrants.

"I don't see its importance because of this," the governor said. "I've always known that my father's father and grandfather and grandmother were from Mexico. I've never denied it. I've always said it."

"Let's just say they did come here illegally. I don't see how I am responsible for that," she said. "I am an American citizen. I am a lawyer. I think it's important to always understand that we are a nation of laws."

The grandfather's "certificate of arrival" lists March 16, 1918, as the official date he was lawfully admitted to the U.S. for permanent residency. He arrived in El Paso, Texas, by traveling on the "El Paso Electric Railway," according to the document.

His "petition for naturalization" contains personal and family information, including the date and place of his marriage and that he had a scar on his right "first finger."

In El Paso, he worked as a taxi driver. The governor said he was estranged from his family of five children, who were born in the city. His wife died in 1934 at age 31, and the children were raised by the wife's mother _ Martinez's maternal great-grandmother.

Historians say immigration between the U.S. and Mexico was largely free of restrictions in the early 20th century. Mexicans could easily declare at checkpoints whether their stay was temporary or whether they intended to become permanent U.S. residents.

The grandfather and his wife paid a "head tax" in July 1918, which was required of immigrants. He obtained a border-crossing card in 1921, making travel easier during World War I, said Marian Smith, a historian at the U.S. Citizenship and Immigration Services.

He was 48 when he became a citizen. It's unclear why the grandfather waited for more than two decades before becoming a citizen.

Smith said many longtime immigrant residents decided to complete the naturalization process after a 1940 law that required the fingerprinting and registration of non-citizens living in the U.S. Another possibility was his marriage in 1941 to a U.S. citizen.

Guadalupe San Miguel Jr., a history professor at the University of Houston and scholar of Mexican-American history, considers it unfair that a Hispanic elected official like Martinez is subject to scrutiny and possible criticism for the immigrant roots of her family.

"It is definitely an anti-Mexican immigrant strain of thought that is being applied to her," he said.

Sabato said questions about a candidate or elected official's family history are fair game in politics, and doubts that she would suffer much damage even if her family had entered the country illegally.

"Maybe it would be more fair if everybody were subjected to the same scrutiny," he said.

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Follow Barry Massey on Twitter at http://twitter.com/bmasseyAP