WASHINGTON — The Air Force Academy is investigating allegations of cheating by 28 freshmen and restricted all 4,300 cadets to campus over the past weekend, asking them to meet in small groups to consider "their self-image and the image of the institution," Air Force officials said.

In addition to the cheating, the academy's superintendent, Lt. Gen. John Regni, cited other recent disciplinary problems in a stern speech last week to the entire student body, faculty and staff.

Forty-three cadets lost their Internet privileges in January because they had downloaded pornography or visited pornographic Web sites, and there have been recurring incidents of alcohol abuse, academy spokesman Johnny Whitaker said.

Nineteen percent of the freshman class was failing academically at the end of the fall semester, chalking up the lowest collective grade-point average in 20 years, he added.

Regni's speech on Wednesday challenged the cadets "to get introspective about this, to look in the mirror — is this the kind of institution that we want to be?" Whitaker said. "He told them he was disappointed with them, that honor is the heart of what we do."

Several members of Congress were briefed on the cheating investigation Thursday during a teleconference of the academy's 15-member Board of Visitors.

Sen. Wayne Allard, R-Colo., said there have been bigger cases of mass cheating at other military academies, including a 1992 incident involving 134 seniors at the Naval Academy, which resulted in the expulsion of two dozen midshipmen.

But Allard said the Air Force Academy is determined to act quickly and firmly, having learned a lesson from two scandals in recent years, one involving sexual assaults on female cadets and the other stemming from incidents of religious intolerance on the Colorado Springs campus.

Restricting all cadets to campus and requiring them to discuss the problems openly is "good preventive medicine," Allard said. "That's how we got in trouble with the sex harassment scandal — they just kept sweeping it under the rug and not dealing with it. This time, they're trying to catch the problems early."

In a brief statement this week, the academy said that 19 cadets so far have admitted cheating on a Jan. 31 "fourth class knowledge test," a routine exam on military affairs given weekly to all 1,200 freshmen. Among the 28 cadets under suspicion of cheating are 19 intercollegiate athletes, the academy said.

The cheating was reported by classmates, and the investigation is being led by cadets, a standard procedure under the academy's honor code.