HARDWICK TOWNSHIP, N.J. (AP) — Firefighters from federal and New Jersey agencies battled a forest fire for a second day Monday in a popular hiking area that is crossed by the Appalachian Trail and a major interstate highway.

The fire burned in New Jersey’s Worthington State Forest and the Delaware Water Gap National Recreation Area. A Facebook post from a nearby fire department in Pennsylvania indicated the fire had grown to 70 acres (28 hectares) by Sunday night.

It still wasn’t clear what the fire’s cause was, but fire officials said it was about 30% contained by late Monday morning. A helicopter and 40 firefighters were working to put the fire out Monday, officials said.

A helicopter dropped water on the fire Sunday, but National Park Service spokesperson Kathleen Sandt told the New Jersey Herald it had to stop operating after sundown for safety reasons.

Volunteer firefighters carried backpack tanks of water to the top of the 1,526-foot (465-meter) Mount Tammany, Sandt said.

Incident Commander Eric Weber told WFMZ-TV the ground crews operating at night had to contend with difficult terrain.

“It’s pretty steep and rugged,” Weber said. “It’s probably the steepest terrain in the entire state of New Jersey.”

The crews were using an existing trail as a fire break, Weber told the station.

Pictures and video shared Sunday on social media showed a wall of flames not far from Interstate 80, which carries traffic to and from New York City and goes through the water gap. It was backed up in both directions.

Some delays were reported during the Monday morning commute, but officials said the highway was open in both directions. They warned, though, that smoky conditions would likely remain until at least Tuesday and flames would also be visible.

The fire broke out on a sunny February day that saw temperatures in the mid-50s. The Appalachian Trail crosses through the recreation area, but it wasn’t clear how close the flames were to it.