U.S. Capitol on lockdown due to "external security threat"

Multiple US Capitol police officers have been shot according to the radio. More police units rush to the complex.

UPDATE: 

A law enforcement official confirms that at least one of the officers was stabbed.

Earlier, a senior Congressional aide and a US Capitol Police source told CNN that after a driver rammed a car into a barricade on Constitution Avenue, the driver exited the vehicle brandishing a knife. 

The US Capitol Police responded and shot the suspect, who was taken into custody, according to police. The sources say the suspect was conscious and taken to a hospital. 

UPDATE - ABC/NBC report the suspect shot by Capitol Police is dead.

UPDATE - USCP says the US Capitol incident is not terrorism-related at this time. One police officer and suspect dead after the US Capitol incident, according to the USCP press conference.

The DC National Guard deployed an Immediate Response Force made up of members of the National Guard to the Capitol complex today to support the US Capitol Police