SNELLVILLE, Ga. -- A mother was shot and killed Sunday in a busy Snellville Target store parking lot.

Police said the victim, identified as 25-year-old Heather Struble, had just picked up her 18-month-old son during a custody swap outside the store at 1905 Scenic Highway.

"The person who dropped the child off had driven away and another person in the parking lot apparently walked over and approached the lady," said Chief Roy Whitehead of the Snellville Police Department.

Witnesses told the police they saw Struble arguing with the man.

SLIDESHOW: Mom Slaying Crime Scene

"They didn't hear specific words, it was just loud and very animated; that is what was reported to us," said Whitehead.

With the child strapped into a safety seat inside the Struble's Chevrolet Tahoe SUV, the man pulled a gun and opened fire.

"He apparently pulled a weapon and shot her in the head and then he walked away," Whitehead said.

Police said the killer was wearing a wig and a fake mustache.

Investigators hope security cameras at the store will give them a look at the gunman.

The child was not hurt and was picked up by relatives.