PHOENIX (Reuters) - Three African refugees were arrested after one tried to board a flight at a Phoenix airport with a fake bomb, in what may have been a test of security, police said on Tuesday.

Luwiza Daman, 51, was taken into custody at Sky Harbor Airport on Friday after security screeners noticed what appeared to be an improvised explosive device inside her carry-on bag as it passed through an X-ray machine, said Phoenix police spokesman Sergeant Steve Martos.

The device, which contained organic material in a container with a cell phone attached, turned out to be a fake explosive, Martos said. Police said they still do not know the nature of the organic material.

Police said that after speaking to Daman, who is from Ethiopia, they traced the object back to Shullu Gorado, 25, and Shani Asa, 34, who are both from Eritrea. Gorado and Asa have since been arrested.

All three are charged with one count each of having a hoax device and conspiracy.

"It could be an innocuous incident, but we wanted to make sure it wasn't a test of security," Martos said. "We have to take everything seriously."

Martos said terrorism has not been ruled out and that the investigation continues. Police said they are still trying to determine the motive behind the incident.

Daman told investigators that she was given the device by Gorado and asked to take it to an individual in Des Moines, Iowa, which was her final destination, police said. Gorado was arrested at his apartment on Friday night.

He told investigators that Asa gave him the device, and he was arrested on Monday and admitted to attaching the cell phone to the container, police said.

An attorney for any of the individuals could not be reached for comment.

It was not immediately known how long the three individuals had been in the United States, but police said they are refugees fromAfrica.

(Editing by Alex Dobuzinskis and Greg McCune)