The incident began near 18th Avenue and Alexander Street around 4:30 p.m. when police responded to reports of shots fired.
When the officer exited his vehicle, the sources say the gunmen who had been shooting at each other, both turned their weapons in his direction.
He returned fire and struck one suspect, killing him.
The other man was also wounded in the gunfire from the initial gun battle and not by police. He was transported to University Hospital in Newark, where he is critical but stable .
Their names are not being released at this time.
The officer involved was not injured.