The car is described as a gray or silver, early-2000's Lexus sedan.
"The driver is described as a young, either in his late teens or early 20s, white male adult," police spokeswoman Heather Graves said.
Xavier was killed in a drive-by shooting in late October, just a couple weeks before what would have been his sixth birthday.
"It breaks my heart, I don't even know how I'm standing right now," his aunt, Asia Thompson said.
Investigators said Monday they believe they have a motive in the case, but would not elaborate further.