FRANKLIN, N.J. -- Burglars climbed a barbed wire fence, rappelled into a museum and smashed display cabinets to steal $30,000 in emeralds, diamonds and other precious stones, leaving a trail of blood throughout the building, police said.

The burglars climbed the fence and forced their way through a second-story window into the Franklin Mineral Museum in New Jersey early Monday morning, investigators said. They rappelled down to the main floor to steal the gems, stones and minerals.

Detectives said the silent alarm first went off at 4:40 a.m., but when police arrived they didn't see anything and left. By the time police came back, five hours had gone by, CBS New York reports.

"There's no way for the officer to have seen any damage to the facility or to have known that anybody had entered the facility based on the entry," Franklin Police Detective Sgt. Nevin Mattessich said.

Get Breaking News Delivered to Your Inbox

Investigators said the glass apparently cut at least one of the burglars, leaving a trail of blood. Police said the injury "appears to be a severe laceration."

The Franklin Mineral Museum tells the story of the Franklin zinc mine in Sussex County. Its mineral exhibit houses about 5,000 items, according to the museum's website.

In 2011, more than $400,000 worth of gold was stolen from the Sterling Hill Mining Museum in Ogdensburg, just a few minutes away from the Franklin museum. The gold was never recovered.

Museum employee Earl Verbeek realized the store had been robbed when he came to work at 9 a.m. He said they stole what looks the most valuable to an untrained eye.

"They went after faceted gemstones and one gold specimen, so it doesn't look like they were mineral collectors," he told CBS New York.