In an unusual advisory, the Anchorage Police Department on Tuesday warned residents of the city not to travel alone at night in "isolated areas," days after an as-yet-unsolved double homicide occurred in a park along a popular trail.

In the statement, police asked citizens "to be cautious" late at night, "especially if they are in isolated areas like our parks, bike trails or unoccupied streets."

"If you do plan to be out late at night, make sure you travel with several friends and not alone," the statement said.

Police did not offer a specific reason why people should not travel late at night alone, other than saying that "criminal activity often increases late at night." The caution came after a wave of homicides in the city, including several unsolved killings. The FBI confirmed Tuesday that they were assisting APD in investigating some of those cases.

On Tuesday, police spokeswoman Jennifer Castro would not say in specific terms what led police to issue the blanket warning.

She said the department "was not trying to create hysteria" with the warning but had seen killings where "we have not connected a motive or a reason as to why these homicides occurred."

The warning also told people to be "extra aware of their surroundings," and to "report any suspicious person(s) or activity immediately to police."

Police so far have consistently declined to say whether they believe any of the victims in recent killings — including two unsolved double homicides on public trails in the past two months — were randomly targeted, or if any of the homicides might be related.

The statement came after three months in which 15 people have been slain in Alaska's largest city. So far this year, 25 people have been killed in Anchorage.

The warning noted that of the 15 people killed in Anchorage since June 27, six were "engaged in high risk behaviors and/or lifestyles" involving drugs, guns or criminal activity, and several were young — 21 or under. Four cases were connected to domestic violence, the statement said.