The thieves had only one thing on their minds when they broke into a Philadelphia library branch early Monday: They left it high and dry, stripped of its copper piping.

Forty personal computers and other valuables were left untouched.

Officials estimate that repairing the damage at the Lillian Marrero branch in North Philadelphia will run about $80,000.

"They took out every ounce of copper piping they could get their hands on," said Siobhan Reardon, president and director of the Free Library of Philadelphia. "Nothing else was taken."

The branch, at Sixth Street and Lehigh Avenue, houses a large technology training center. The food agency Philabundance and the city Department of Licenses and Inspections have offices in the basement.

The library will be closed for at least two weeks as the pipes are replaced, Reardon said.

An alarm was tripped about 2:45 a.m. Monday, Reardon said. Burglars cut through 15 wrought-iron bars on a grate that covers an oversize rear window. They then bent the bars back.

"How did they do that?" Reardon asked.

Once past the grate, they broke the window, slipped into the basement, cut out the pipes, and removed the insulation that wrapped them.

"We figure they knew enough to shut the water off before they did their deed," Reardon said.

The theft was discovered when library staff arrived for work and found the water was not running, Reardon said.

Since the break-in, plumbers have restored some running water to the Philabundance offices - "a Band-Aid measure," Reardon said - but completing the repairs will take months.

The theft is the second at the Marrero branch in less than a month. Over the July Fourth weekend, thieves stole copper pipes running to an air-conditioning unit behind the building.

Contact staff writer Sam Wood

at 215-854-2796 or samwood@phillynews.com.