Izumi Sakai, singer in the pop group ZARD, died of a brain contusion Sunday at a Tokyo hospital after falling down a stairway, her office said Monday. She was 40.

Police are investigating to determine whether her death was accidental or a suicide.

Sakai had been repeatedly hospitalized since last June due to cervical cancer, her office said. She returned to the hospital in April after the cancer was found to have spread to a lung, it said.

A native of Kanagawa Prefecture whose real name was Sachiko Kamachi, Sakai made her singing debut in 1991 with ZARD. She wrote lyrics and gained popularity with young people with such songs as "Yureru Omoi" ("Swinging Feeling") and "My Friend."

According to police and her office, Sakai fell about 3 meters down an emergency staircase in Keio University Hospital early Saturday, hitting the back of her head.

"She slipped on the staircase due to rain the previous day when she was going back to her hospital room after a walk," her office said.