President Bush Wednesday said he was "stunned and not happy" about the Immigration and Naturalization Service's belated dispatch of student visa approvals for two of the terrorists who slammed hijacked jets into the World Trade Center on Sept. 11.

After reading about the visa notices, which turned up six months after the attacks, Bush said he was "plenty hot."

He called the episode a "wake-up call for people who run the INS," an agency he said "needs to be reformed."

Bush made the comments at an afternoon press conference, shortly after ordering Attorney General John Ashcroft and Homeland Security Director Tom Ridge to "get to the bottom of this immediately."

Ashcroft, calling the incident "inexcusable," directed Justice Department Inspector General Glenn A. Fine to find out why immigration officials failed to intercept the notification letters and why there was such a long delay in processing them.

On Monday, as memorial services were held throughout the U.S. to commemorate six months since the Sept. 11 terror attacks, a Florida flight school received paperwork from the INS confirming that it had approved visas for Mohamed Atta and Marwan Al-Shehhi.

Atta, 33, and Al-Shehhi, 23, were aboard the airliners that smashed into New York's Twin Towers, killing thousands. The pair trained at Huffman Aviation in Venice, Fla., in 2000 and early 2001 and sought student visas so they could attend technical schools.

Atta and Al-Shehhi were cleared to stay in the United States until Oct. 1, 2001, according to the forms.

"When they hit the buildings they were approved to be here," said flight school owner Rudi Dekkers.

Dekkers told Fox News he was irritated that about 10 minutes after Bush said he was going to investigate the INS over the visas Dekkers received for the two hijackers, an INS agent came into the flight school asking for the visas back.

The INS agent, in casual clothes, came in and told Dekkers he could give them over or be subpoenaed for them. Dekkers said he'd give them up, and then decided he wanted to be subpoenaed instead to show proof he'd had them and given them up.

Dekkers said "I feel very awkward now" that the government got those visas back by coming in with a subpoena.

The visa for Atta, of Egypt, was approved in July 2001. Al-Shehhi, of United Arab Emirates, was approved the following month.

But the paperwork containing their student visa approval forms from the INS did not arrive until this Monday.

INS spokesman Russ Bergeron said the embarrassing delay was due to a backlog of documents at a federal paperwork processing center in London, Ky.

He described the paperwork as "backup notification." He said the INS had already notified the men and the school last summer about the approvals.

Atta and Al-Shehhi initially entered the U.S. on visitors' visas. They applied for M-1 student visas, given to immigrants attending technical schools. Foreign students are generally allowed to study in the U.S. as long as they apply for student visas, Bergeron said.

"The important thing to recognize is the decisions to change their status were made ... before Sept. 11, and at the time there was no information made available to INS regarding these people and their link to terrorism," he said.

Michigan Rep. John Conyers, the top Democrat on the House Judiciary Committee, said the notification was a sign of what he called the Bush administration's "misguided focus in pursuit of homeland security."

"I am astonished that while the INS is fixated on detaining and rounding up countless Arab-Americans without any justification, it has failed to take basic steps to ensure that visas are not issued to known terrorists," Conyers said.

Atta and Al-Shehhi studied at the Technical University in Hamburg, Germany, received pilot training at Huffman and practiced their flying on a Boeing 727 flight simulator in the Miami suburb of Opa-locka.

The Associated Press contributed to this report.