WASHINGTON (CNN) -- Two intruders bolted through a security checkpoint Thursday afternoon at the House Cannon Office Building after an X-ray machine showed what looked like a revolver in a knapsack, a Capitol Hill police spokeswoman said.
The security breach prompted police to begin evacuating the building and systematically searching each office for the intruders, described as a white man and white woman in their 20s.
It was unclear whether the detected revolver was a real gun or a toy.
"The possible weapon was a .38. We do believe that it is a revolver, " Capitol police spokeswoman Jessica Gissubel said.
Authorities are looking for a white man described as 5 feet 3 and 140 pounds, she said. He was wearing a black suit and white shirt and carried a black and grey backpack.
The woman was wearing khaki pants, a pink shirt and sunglasses atop her head, Gissubel said. She is described as 5 feet 5 with long brown hair extending past her shoulders.
"The Capitol police have put the building on lockdown. We are going to be letting people exit the building, as long as they agree to an administrative search," Gissubel said.