Three African immigrants to the U.S. have pleaded not guilty to what has been described as a fake bomb incident at a U.S. airport earlier this month.

The suspects, who are from Eritrea entered the pleas Friday in a federal court in Arizona.

Officers checking a carry-on bag found a cell phone taped to a candy box. Prosecutors say it looked like an explosive device.

The suspects said the phone was a gift for a relative they were going to visit and that it was taped because the phone was broken.

Although no bomb was involved, the ability to move a replica of an explosive device past airport security has raised concerns about airport safety. The Federal Bureau of Investigation described the cell phone incident as a possible test of security.

The suspects remain in custody, pending a hearing next week for two of them. A judge set their trial for October 4. Each suspect faces up to five years in prison and a $250,000 fine if convicted.