HERNDON, Va. - Northrop Grumman Corp. said on Thursday that it issued layoff notices to 180 employees at its Hagerstown maintenance and modification center. Northrop said the notices went out because it expects a lull in work at the center. The company said it expects to finish current projects with the Navy and U.S. Customs and Border Protection in mid-June. The employees will remain at work at least until then, Northrop said. The company said it might have new work at the facility in the early fall. He said the company is looking into using the employees at other Northrop sites.