It happened just after 11 a.m. at the restaurant location on Westfield Avenue in Elizabeth.
Officials say that some landscapers were working on the property and using a substance that got into the electrical system.
Then, an employee tells Eyewitness News that a worker was plugging something in when there was an explosion and his back was burned.
One of the landscapers also suffered burns on his hand.
There was thick smoke inside the restaurant, and four people inside made their way out.
The victims were taken to the hospital for treatment.
The extend of their injuries is unknown.
Firefighters arrived and extinguished the two-alarm blaze.
The fire damage is near the back of the fast food restaurant, where investigators were trying to find the cause.
ALSO READ | Ghost guns: Eyewitness News investigates a growing menace
* Get Eyewitness News Delivered
* More New Jersey news
* Send us a news tip
* Download the abc7NY app for breaking news alerts
* Follow us on YouTube
Submit a News Tip