Six African teenagers who competed in the international robotics competition in Washington, D.C. this week have vanished, and two of the teens were seen crossing into Canada, D.C. police say.

The members of the robotics team from the eastern African country of Burundi, who are 16, 17 and 18, disappeared Tuesday after they took part in the First Global Challenge robotics competition.

The Metropolitan Police Department has received reports that Audrey Mwamikazi, 17, and Don Ingabire, 16, were seen crossing into Canada, spokeswoman Aquita Brown said Thursday morning.

Police say they have no indication of foul play in their disappearance. No additional details were released immediately.

The teens range in age from 16 to 18 and include four males and two females. The D.C. Police Department identified them on Twitter as: Audrey Mwamikazi, 17; Aristide Irambona, 18; Kevin Sabumukiza, 17; Don Ingabire, 16; Nice Munezero, 17; and Richard Irakoze, 18.

The competition, designed to encourage youths to pursue careers in math and science, attracted teams of teenagers from more than 150 nations. A team of girls from Afghanistan drew international attention after they were twice rejected for U.S. visas and President Donald Trump intervened.

Competition organizers say they learned Tuesday night that the team’s mentor could not find the students. First Global President Joe Sestak made the initial call to police, according to a statement issued Thursday.

“Security of the students is of paramount importance to First Global,” the statement says. “First Global ensures that all students get to their dormitories after the daily competition by providing safe transportation to the students staying at Trinity Washington University who are always to be under close supervision of their adult mentor and are advised not to leave the premises unaccompanied by the mentor.”

The team’s mentor said they disappeared after the competition. He said he did not know where they went. The mentor told police the teens have one-year visas to stay in the U.S.

Police reports say investigators tried to contact the uncle of one missing teen but got no response. The reports say police canvassed DAR Constitution Hall, where the competition was held.

The investigation is ongoing.

The competition’s webpage about Team Burundi shows the six team members posing with the country’s flag and says the youth are the top-achieving students in the capital city, Bujumbura.

The team’s slogan in Kirundi — the country’s official language, along with French — is “Ugushaka Nugushobora,” meaning “where there is willing, [there] is also the ability.”