Two Coast Guard members were fatally shot at a communications station on Kodiak Island off Alaska, officials said Thursday.

Officials believe a third person was involved in the shooting, which appears to be a double homicide, but no one was in custody as of Thursday afternoon.

The communications station has 59 staff members and people who want to enter must show identification, said Coast Guard Petty Officer Charly Hengen.

"It is a secure facility," she said.

The station serves as the "ears in the sky" for radio transmissions from mariners and aircraft, Hengen said, and is responsible for relaying distress calls to the air station of the Coast Guard in Anchorage.

There is a commanding officer and enlisted and civilian personnel at the station, which is about eight miles from the island's largest city of Kodiak, Hengen said.

Moore told the city's 6,300 or so residents "to remain vigilant and to report any suspicious activity to local law enforcement officials" until more details emerge.

The Guard said the identities of the victims would be released after family members were notified. They did not yet know whether the shootings took place Wednesday or earlier Thursday.

The FBI said agents were headed to Kodiak from Anchorage about 250 miles away.

"We are deeply saddened that we lost two shipmates," said Moore. "This is a rare occurrence and we are going to do everything possible to ensure we find out exactly what happened."

The Associated Press contributed to this report.