It happened just after 5 p.m. at the Camden Westwood Apartments at 2500 Summit Ridge Loop.
Officials said the fire was put out in 30 minutes by about 30 firefighters, with Apex, Cary and Western Wake Fire Departments assisting.
According to the Town of Morrisville, 35 people and 9 pets were displaced. The fire department said about 17 apartments were affected by the fire.
No injuries were reported.
On Thursday, the Town of Morrisville said the cause of the fire was found to be "undetermined."
Michael Giangrasso's apartment took the brunt of the fire. He had just left to walk his dog.
"When I took Jesse out, I looked around the rest of the apartment, I didn't see any smoke," Giangrasso said. "I came rushing home and our place was completely gone."
He said he noticed a strong scent from cooking but has no idea if that's where the fire came from
"It happened in 15 minutes," Giangrasso said. "I would say that's it and it was just devastating but I got out with my fiancé, I got out with my dog. When these things happen, it really makes you appreciate what's important in life: it's not the things. It's the people and everything else that you have like the dogs.
"There's a lot of stuff we lost obviously it looks like our apartment took one of the biggest hits but I look down at my dog and I look at my fiancé and that's what is important so it's finding solace in just that," he added.