6:29 PM PT -- 5/8 -- A law enforcement source tells us the whole incident started after the woman was told by a Walmart employee to put on a face mask. The woman allegedly refused and a verbal spat started ... the officer then got involved after things got heated and took the woman down when she started to resist.
An off-duty Birmingham, AL cop working security at a Walmart is under investigation after body-slamming a customer for allegedly refusing to wear a face mask.
The incident went down Tuesday, and the video starts with the security guard struggling to detain the woman. Moments later eventually he lifted her from under her legs and slammed her to the ground.
A nearby woman, also not wearing a face covering, was pissed and shouted at the security guard for slamming her friend's head on the ground.
The video, posted online, prompted Birmingham PD to launch an investigation into the use of force.
For what it's worth ... Alabama's stay-at-home order only recommends -- not requires -- residents to wear face coverings while out in public. We called the store, who said they do require shoppers to wear face coverings inside.
A Birmingham PD spokesman said, "Our records indicate we have not issued any citations or made arrests as it relates to the shelter in place ordinance or the face covering ordinance. We have made a valiant effort to gain voluntary compliance and we will continue to work cohesively with the community."