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A four-unit apartment building at 690 Wilbur Ave. was significantly damaged by fire just after 8:30 a.m. Wednesday in Phillipsburg.
Emergency personnel went to Thomas Street and Wilbur Avenue as flames poured from windows to the rear of the yellow-sided structure.
No one was hurt, fire Chief Richard Hay said. Warren County fire Marshal Joe Lake and town police were there as the investigation into a cause and ignition site were beginning about 9:45 a.m.

The upper floor to the rear appeared to sustain the most damage. There are three apartments in the main portion of the building with a studio nearest where the flames were seen, a tenant of another apartment said.
Frederick Newman wasn't having the best of 35th birthdays. He was called at work and alerted about the fire. His apartment is in a one-story extension on the left side of the building the farthest from the fire. He wasn't sure whether his unit was smoke or water damaged.
Windows were broken out on the second floor to the front of the main structure. A hole was cut into the roof. A chain saw about 9:30 a.m. was digging through the attic wall nearest where the fire had been.
Newman said he saw the resident of the studio apartment on Tuesday night but hadn't seen her Wednesday morning.
He said he didn't know much as he stared from behind a firetruck across Wilbur Avenue from his home.
"I have no clue what's going on," he said, adding he hadn't been allowed to look into his apartment, where he's lived for five years. He said he doesn't have renter's insurance.
The building is owned by Brian and Sheri Nehlia of Forks Township, property records show. It was built in 1940.
Hay said he would try and release more information later in the day.
Tony Rhodin may be reached at arhodin@lehighvalleylive.com. Follow him on Twitter @TonyRhodin. Find lehighvalleylive.com on Facebook.