WASHINGTON -- The only Chinese-American member of Congress says he was refused entry at the Department of Energy by security guards who repeatedly asked him if he was a U.S. citizen.

Department spokeswoman Jeanne Lapatto denied that Rep. David Wu, D-Oregon, was asked repeatedly if he was a citizen.

Wu had been invited to address department employees Wednesday during Asian Pacific American Heritage Month. Wu said he was denied access even after showing his congressional identification.

Nevertheless, Energy Secretary Spencer Abraham showed up unexpectedly at Wu's office Friday to apologize, an aide to Wu said. The aide said the apology was not specific, but Abraham told Wu his agency would conduct an internal review, including a look at discrimination.

Lapatto said that as a security measure, all visitors to the department are asked if they are U.S. citizens when they check in upon arrival. She said a preliminary investigation into Wu's complaint found that there was a "mix-up" involving the location where the congressman was supposed to meet his escort, and that the confusion led to his delay getting into the building.

Lapatto said that when the guard was calling around the building to find the escort for Wu, the guard was repeatedly asked over the phone whether or not Wu was an American citizen. She said the guard repeatedly responded that he was and that the congressman probably heard the question answered again and again.

"I think there was some miscommunication there and they were trying to work some things out," Wu told CNN Friday evening, "but they did ask me about my citizenship after I showed them the congressional ID."

Wu said he was asked three times about his citizenship.

Wu, who sits on the Energy Subcommittee of the House Science Committee, deals regularly with issues of discrimination against Asian-Americans and has taken a special interest in concerns by DOE lab employees concerned about the federal government's handling of the Wen Ho Lee case, press secretary Holly Armstrong said.

Wu sent a letter to Abraham on Thursday to complain about the way he was treated and the signal that treatment sends to Asian-Americans.

"The conduct of the DOE guards is both ironic and disturbing," he said. "Ironic because I was invited by DOE to speak about the progress Asian Pacific Americans have made in America. My citizenship has never been questioned at the White House, the Supreme Court or in the U.S. Capitol -- all locations with potentially sensitive information."

Wu said he is disturbed that the incident may represent "the tip of the iceberg" for Asian-Americans employees and potential employees at DOE.

"My understanding is some of the brightest graduate students in the country, who happen to be Asian-American, are refusing to go to work for the Department of Energy," Wu said in a statement on the House floor Friday.

"I am going to encourage the Department of Energy to redouble its efforts and engage in a true process of soul searching," he said. " Do you really ask everyone their citizenship at the door? And if so, is that an effective way of enhancing national security?"

Lapatto said Abraham, who as an Arab-American has suffered from discrimination in the past, issued a memo shortly after taking office, asking all DOE managers and employees to be careful that racial profiling does not exist in the Department.

"It was a mix-up. We're sorry," Lapatto said. "We're not trying to hurt anyone's feelings. It was just a mix-up."