MINNEAPOLIS — For 14 years, Isaac Owusu’s faraway boys have tugged at his heart. They sent report cards from his hometown in Ghana and painstaking letters in fledgling English while he scrimped and saved to bring them here one day.

Letters from boys in Ghana whom Isaac Owusu considers his. He will petition as a stepfather since tests showed three are not related to him. So when he became an American citizen and officials suggested taking a DNA test to prove his relationship to his four sons, he embraced the notion. Imagine, he marveled as a lab technician rubbed the inside of his cheek, a tiny swab of cotton would reunite his family.

But modern-day science often unearths secrets long buried. When the DNA results landed on Isaac Owusu’s dinner table here last year, they showed that only one of the four boys — the oldest — was his biological child.

Federal officials are increasingly turning to genetic testing to verify the biological bonds between new citizens and the overseas relatives they hope to bring here, particularly those from war-torn or developing countries where identity documents can be scarce or doctored.

But while the tests often lead to joyful reunions among immigrant families, they are forcing others to confront unexpected and sometimes unbearable truths.

For Isaac Owusu, a widower, the revelation has forced him to rethink nearly everything he had taken for granted about his life and his family.

It has left him struggling to accept what was once unthinkable: that his deceased wife had long been unfaithful; that the children he loves are not his own; and that his long efforts to reunite his family in this country may have been in vain.

The State Department let his oldest son, now 23, come to the United States last fall, but said the others — a 19-year-old and 17-year-old twins — could not come because they are not biologically related to him.

Isaac Owusu, who asked that only his first and middle names be published because he would like to keep his family’s pain private, is still hoping the government will allow the teenagers to join him, arguing that he has been a devoted stepfather, if not a biological parent.

But in recent months, he says, he has simply unraveled.

“Sometime when I get in bed, I don’t sleep,” said Isaac Owusu, 51, who works for an electrical equipment distributor and an auto supply shop.

“I say to myself, ‘Why this one happen to me?’ ” he asked, his eyes wet with tears. “Oh, mighty God, why this one happen to me?”

A similar sense of shock is reverberating through other families across the country as genetic testing becomes more common. State Department and Homeland Security Department officials do not keep statistics on the number of DNA tests taken by new citizens or permanent residents, who are allowed to bring some close relatives to the United States if they can document their family ties.

But Mary K. Mount, a DNA testing expert for the A.A.B.B. — formerly known as the American Association of Blood Banks — estimates that about 75,000 of the 390,000 DNA cases that involved families in 2004 were immigration cases. Of those, she estimates, 15 percent to 20 percent do not produce a match.

Negative results can suggest an effort to bring in illegal immigrants or distant relatives, officials say, though they note that requests for DNA tests deter illicit activities. An official, who spoke anonymously because he was not authorized to discuss the cases, found no indication of wrongdoing by the families interviewed for this article.

Such genuinely unexpected results hit immigrant families particularly hard because DNA testing sometimes provides the best chance of reuniting with loved ones abroad.

“Sometimes these are complicated families,” said Tony Edson, a deputy assistant secretary of state. “People are learning things that they never knew about themselves.”

In California, for example, a Mexican-American family splintered after a DNA test showed that a young woman, a new citizen, was not related to the man she considered her father. The man, who was living in the United States, was ordered back to Mexico because his visitor’s visa had expired.

In Maryland, a man from Sierra Leone discovered that his baby back home was the product of a hidden trauma. His wife, who was separated from him during their country’s civil war, had been raped by rebels. In her shame, she had never revealed the truth.

New citizens and permanent residents are asked — not required — to take the tests if they lack documentation of ties to relatives overseas. Physicians designated by the State Department typically collect samples from relatives abroad and send them to this country for testing.

A negative result does not eliminate the possibility of reunification. New citizens can adopt children under 16 and bring them to the United States, officials say. They can also petition for stepchildren or stepparents in certain circumstances.

But immigrants say officials rarely notify them of such alternatives. Meanwhile, lawyers say the government’s growing reliance on DNA testing burdens immigrants who often pay $450 or more to test parent and child.

Officials counter that the process helps reunify families who might otherwise remain divided because they lack adequate documents. But they acknowledge that genetic testing can carry an emotional toll.

Tamara Gonzalez, a new citizen from Jamaica, said her test result has forced her to question her very identity.

She and her father, who lives in Jamaica, took the tests last year after she applied to bring him to the United States. When she learned they were not related, she confronted her mother, who said the result must be a mistake.

Mrs. Gonzalez, who works at a day care center in Brooklyn, said she would like to believe her mother. But she said her faith in her family bonds had been shaken. “It changes my sense of who I am,” said Mrs. Gonzalez, who is 31. “And it has changed things between me and my mother.”

“I wonder now if there’s something she’s hiding or not saying,” she said. “I start to wonder: Who is my father? Am I ever going to know?”

Clevy Muir, the man she knows as her father, says he is still trying to sort out their options.

“I’m not going to give up my daughter, you understand?” he said. “But where can I turn?”

Balfour Francis, a 44-year-old Jamaican-born welder in Brooklyn, had even set aside a bedroom for the teenager he considers his daughter. She was born to a woman he had never married, but he had never doubted that she was his baby girl.

Then came last year’s DNA results. Now, he said, the bedroom is used for storage while he struggles to get immigration officials to tell him what he can do next.

“I will not let anybody dictate who is my child,” said Mr. Francis, who is a permanent resident and has a wife and children in New York. “I try to assure her I am who I will always be.”

Meanwhile, Isaac Owusu cannot keep the faces of his boys in Ghana out of his mind.

They call him collect on weekends, begging him to explain why he left them behind. At night, he sees them in his dreams with those big brown eyes that everyone used to say resembled his own.

“They ask me, ‘Why? Why? Why?’ ” he said. “ ‘You come and pick up our senior brother. What about us?’ ”

He blames the bureaucracy for the delay because he cannot bear to tell the truth. They are already motherless, he said. How can he tell them they are fatherless now, too?

Over the years, while his sister cared for the boys, he has sent money for tuition and uniforms, doctors and food. He has saved their letters. (“Father, in Ghana we are in the rainy season so I need two thing,” one son wrote, “rain coat and rain boot.”) He has pored over their report cards (“Obedient and respectful,” one teacher wrote), urging them to study harder so they could succeed here.

He moved, with a new wife, from an apartment to a house to make room for them all, and became a citizen in 2002. But last year’s DNA tests dashed his hopes for a speedy reunion.

After months of inquiries, Elizabeth M. Streefland, his immigration lawyer, finally determined that he could petition for the teenagers as their stepfather. He must prove that the boys are the children of his deceased wife. Isaac Owusu hopes that a DNA test of one of his wife’s siblings, which could be compared with that of the teenagers, would provide that proof.

That will cost more money. But he says he simply cannot give up on his boys. “I tell them, ‘Daddy still loves you,’ ” he said. “ ‘Anything it takes, I will do to get you over here.’ ”