Honolulu police say they are investigating a “serious hit-and-run motor vehicle collision” in which a male motorcyclist crashed in Waialua Sunday night, leaving his injured passenger behind.

According to a Honolulu Police Department report, at about 7:10 p.m., the unidentified motorcyclist was traveling west on Waialua Beach Road with a 33-year-old woman passenger, when he traveled onto the unimproved shoulder and collided into a plastic water barrier.

The two were ejected and the motorcyclist “fled the scene leaving his passenger behind,” police said.

Honolulu Emergency Medical Services paramedics treated the woman and took her to a hospital in serious condition. She was not wearing a helmet, according to police.

The motorcycle was described as a dark-colored Harley Davidson. Police said it is unknown if speed, drugs, or alcohol were factors in the crash.

Anyone with information about the incident is asked to call HPD’s Traffic Division at 808-723-3413.