A group of student volunteers from the Northern California who had been robbed at gunpoint said Sunday they planned to continue their aid efforts despite the attack but will be accompanied by Guatemalan police as they continue their trip.

The group of 13 teenagers and 4 adults, mostly from Sonoma and Marin counties, had been traveling by bus Friday on their way to help build an elementary school in El Salvador (search) when their bus was commandeered by gunmen about 78 miles west of Guatemala City (search).

The assailants fired shots in the air and robbed the volunteers of more than $11,000 and other belongings. No one was hurt in the attack, which the group reported to police in Antigua and U.S. Embassy officials.

Despite the setback, the expedition led by the Sonoma nonprofit Seeds of Learning will continue on to El Salvador, trip leader Annie Bacon said Sunday.

Guatemalan police guards accompanied the group on Sunday, as the volunteers stopped to make phone calls to parents and family members.

Although robbers made off with the money that had been donated for the service project, new donations have arrived at Seeds of Learning (search) as word spread of the robbery.

Only one member of the trip is considering returning to United States immediately -- for reasons unrelated to the robbery -- Bacon said.

"They understand more than ever how much help means," said Bacon, who used a credit card Sunday to withdraw cash for the group.

Guatemalan authorities plan to escort the group to the El Salvador border on Monday, Bacon said. Arrangements also have been made for a security escort in El Salvador.

Bacon said members of the expedition were more determined than ever to reach El Salvador, where they hope to work on a school that still has no roof.

Seeds of Learning, now 12-years old, takes about a dozen groups of volunteers each year to Guatemala, Nicaragua and El Salvador to build schools in rural communities. The 17-member group had spent the first week of its three-week trip studying Spanish in Guatemala.

A U.S. citizen was killed during a similar attack on U.S. travelers in Guatemala in January.

Thirteen members of the Church of Jesus Christ of Latter-day Saints, or Mormons, most from Ogden, and Salt Lake City, Utah, were traveling from the mountain city of Quetzaltenango to the Mexican border on Jan. 7 when men opened fire with automatic weapons, forcing their bus to stop.

Brett Richards, a 52-year-old architect from Ogden, was shot in the chest and died on the way to a hospital. The robbers led the other passengers into a forest and forced them to lie face down before stealing their belongings.