A video that went viral over the weekend shows what appears to be a young mother nodding off on a Philadelphia bus while a little girl calls out and tries to wake her.

Officials speculate that drugs may be involved -- that the woman may be doing what's known as the "heroin nod" -- with the little girl in her care. But they also want to know why no one took action until the video was posted on YouTube.

"When you have a person doing the 'heroin nod' and a little girl is trying to hold her head up, that's horrifying," Tom Nestel, chief of police for the Philadelphia-area public transit system SEPTA, told Philly.com. "There's very little reason why 15 calls to 911 weren't received. I don't think there's anyone who can watch that and say, 'Ah, that's a shame,' and just walk away."

Department of Human Services (DHS) Commissioner Anne Marie Ambrose echoed those remarks.

“While it is helpful that so many people emailed and called us after viewing the video, there were many people on the bus who witnessed this as it was occurring and took no action at all,” Ambrose told NBC Philadelphia. “Child abuse is a community problem, if you see or know that a child is being abused or neglected, you should report it immediately.”

Philadelphia Police Lt. John Stanford says in situations like this, it's best not to confront the person. Instead, alert the bus driver or call 911.

The Philadelphia Police Department's Special Victims Unit and DHS are investigating the incident, and have questioned the woman, whose identity has not been released.

At the time of this writing, no charges have been filed, according to Philly.com.

WARNING: The video that sparked this investigation contains disturbing content.