A 77-year-old woman who was brutally beaten as she got off a Muni bus earlier this month died last week, and police say that they're still not sure why she was attacked.

San Francisco Police Department spokesperson Officer Grace Gatpandan says that the victim, Lin Leung, frequently rode the 2-Clement, 3-Jackson, 30-Stockton and 45-Union/Stockton Muni lines on her way between Japantown and Chinatown.

Leung had just completed one of those trips, exiting a bus near Stockton Street between noon and 2:30 p.m. on Sunday, May 10. That's when a random attacker hit Leung at least twice in the head, causing her to fall to the ground, Gatpandan says.

A bystander helped Leung up and away from the bus, Gatpandan says, but left before police arrived.

In a May 18 Facebook post, Leung's grandaughter, Connie Ho wrote "On May 10th, Mother's Day, my grandma was struck in the head by an African American woman in SF Chinatown. My grandma is now in a coma in ICU and will never regain consciousness to see her family again. She is leaving our family very soon and I ask of your help to come forward if you or anyone you may know that witnessed this. To the Good Samaritan that helped my grandma, please reach out to us. Any information will be of great help."

Leung was pronounced dead at San Francisco General Hospital on Friday, according to the medical examiner’s office.

Nothing was stolen in the attack, Gatpandan says, and a motive for the beating remains unclear.

Police say that there were surveillance cameras in the area, but at publication time they did not have a description of the suspect or any additional details in the assault.

Gatpandan says that police are seeking witnesses to the attack, including the Good Samaritan who helped Leung. Anyone with information on the case is urged to contact SFPD Sgt. Trevor Kelley at trevor.kellysfgov.org or through the Anonymous Tip Line at (415) 575-4444 or by texting the tip to TIP411 with “SFPD” at the start of the message.

At publication time, the ME's office had not confirmed if Leung's death would be ruled a homicide. If it is, it will be San Francisco's 21st for 2015, following the drive-by shooting of 30-year-old Ramon Cruz on May 9.