SAN ANTONIO ? President Bush strongly defended his domestic spying program on Sunday, calling it legal as well as vital to thwarting terrorist attacks, and contended the leak making it public had caused “great harm to the nation.”

“This is a limited program designed to prevent attacks on the United States of America and, I repeat, limited,” Bush told reporters after visiting wounded troops at Brooke Army Medical Center.

The New York Times reported last month that the National Security Agency had been conducting warrantless surveillance since 2002. Bush then acknowledged that he had authorized the NSA program and pointed to informing congressional leaders and regular reviews by administration officials as evidence of oversight for the program.

The Justice Department on Friday opened an investigation into the leak.

“The fact that somebody leaked this program causes great harm to the United States,” Bush said before returning to Washington. “There’s an enemy out there.”

Bush stressed that the surveillance involved telephone calls from “a few numbers” outside the United States by people associated with al-Qaida, the terrorist organization that plotted the 9-11 attacks. The White House later clarified Bush’s remarks, saying he meant to say calls going to and originating from the U.S. were being monitored.