A US State Department official said the department was aware of reports that a US citizen went missing on Rendezvous Caye.
Mackenzie is a mother of two and lives in Prescott, CNN Phoenix affiliate KPHO/KTVK said.
Hilberto Romero, deputy head of the Belize National Crimes Investigation Branch, told local news Channel 5 Belize that Mackenzie was last seen walking around the island about 10 p.m. Friday.
On Saturday morning, her boyfriend reported her missing, Romero told the station.
Mackenzie traveled to the Central American country in early December with her boyfriend, according to police.
Raggamuffin Tours, the company that operated the excursion, confirmed the incident in a Facebook post and said it was cooperating with local authorities.
“We are in contact with and have offered our full cooperation to the family of Alison Mackenzie, and we assure them that we will continue do all in our power to assist law enforcement in their investigation into her disappearance,” the post said.
Police told CNN they questioned other tourists on the trip and the investigation was ongoing.
Mackenzie’s sister, Jodie Dixon, told 7 News Belize she did not believe alcohol played a role in the disappearance,
“I will say it’s not like her to be intoxicated,” Dixon told the station. “Maybe have a drink, but to be intoxicated, like I’ve never seen her drunk in my life.”
The missing woman’s mother, Cherie Mackenzie, appealed on Facebook for help. “My whole family is beyond devastated, it just doesn’t seem real,” she wrote. “Her 2 daughters are past being consoled. They have lost their Mother and best friend. I want her home with us, no matter how she may be found. I want my daughter back.”
Ragamuffin Tours declined comment.
CNN’s Jennifer Henderson and Jennifer Hansler contributed to this report.