CNN's Kelli Arena reported that heavily armed officers were headed to the area.

The alert came moments after a press conference held by Rep. John Sensenbrenner on immigration reform, CNN producer Deidre Walsh said.

"We saw several Capitol Hill police officers running quickly down the hallway," she said. Soon after that an announcement was made throughout the building.

CNN's Ted Barrett said people were moving outside the Rayburn about an hour after the report came in. That was "probably a good sign," he said.

Five ambulances, "a ton of police cars and a ton of cops" were also there, but there was no indication of any immediate threat, Barrett said.

"It certainly is calm and controlled at this point," he said. "The police do not seem overly concerned that there might be a gunman loose at this point."