GUATEMALA CITY (AP) _ Gunmen ambushed a bus carrying 16 American college students and teachers on an isolated Guatemalan highway. Marched at gunpoint to a nearby field of sugar cane, all were robbed and five were raped during the 90-minute assault.

The group from St. Mary's College in Maryland returned home following the attack near Santa Lucia Cotzulmalguapa, a hilly region known for banditry about 40 miles from the Guatemalan capital.

Two men, ages 37 and 24, were in custody today in connection with the assaults, an official in the criminal investigations unit of the national police told The Associated Press on condition of anonymity.

The students, 12 women and one man, were returning to Guatemala City on Friday afternoon with two male faculty members and a female administrator after an educational tour of historic and cultural sites.

The gunmen held the group for 1 1/2 hours before fleeing as local police arrived, the president of the college, Jane Margaret O'Brien, said during a press conference today at the college.

In two previous trips to Guatemala, St. Mary's students had encountered no problems, she said. And prior to this tour, the school had been in touch with U.S. Embassy about safety in the country.

The U.S. State Department does not warn American citizens against traveling to Guatemala, but its consular information sheet does note that crime has been increasing in the country.

``There was not an indication to us that we were at greater risk than at other times,'' said O'Brien.

The area surrounding the site of Friday's attack, once rife with guerrilla activities, has a reputation for lawlessness. Guatemala has been shaken the past year or so by a wave of kidnappings, roadblock assaults and a surge in crime since the 36-year civil war ended.

The increase in violence first drew national attention when 22-year-old college student Beverly Sandoval Richardson was kidnapped in May 1996. Following six months of public appeals by her family and a fruitless search by police, her body was discovered in a shallow grave. Twenty people were arrested in connection with her death.

The students who were raped were treated at Herrera Llerandi Hospital in Guatemala City and returned Saturday to the United States. The remaining students returned to Washington's Dulles airport Sunday night and were met by O'Brien and the college's dean of students.

``We are heartbroken by this senseless violent attack on our students and colleagues,'' O'Brien said in an earlier statement.

The college has been working with the State Department, which is treating it with ``the utmost urgency,'' Meringolo said.

The State Department official said that the U.S. government is working with Guatemalan authorities.

The tour was sponsored by the public four-year liberal arts college in St. Mary's City, about 70 miles southeast of Washington. It was the third year that the college, which has 1,500 students, had sponsored an anthropological tour to the Central American nation.

Meringolo refused to identify any of the staff members or students.

Jeff Mohler, a senior at St. Mary's, said today he was stunned by the news of the attack.

``Nothing ever happens here,'' said the 21-year-old. ``The worst thing is a late-night party and something gets broken.''

Former Maryland Gov. William Donald Schaefer, a college trustee, called it ``a terrible experience.''