No, it is not paranoia. Crazy people go on shooting rampages in the US all the time, and the carnage happens in minutes. Security should be the most tight at and near the Capitol. The President must be looked after at all times. In fact, any suspicious people should be stopped and either interrogated or searched. There should be a bill passed making these searches legal. There is a honor system in place in society. If people see wrongdoing, they get on their cellphones and call the authorities -- oftentimes people will videotape a criminal act also. It's better to be safe than sorry.
The capitol must be kept under incredibly tight security. Anything seen or heard must be reported and taken care of immediately, and as with any situation, there is the potential that it can escalate. In a place as sensitive as the United States capitol, it is important to keep open eyes and stay aware of the surroundings continually.
This country has become a dangerous place and no one knows where and when the next mass shooting will take place. Although not every incident causing a lockdown or law enforcement response is a dangerous one, it is important to be vigilant. The inconvenience of unnecessary lockdowns and police responses is outweighed by the fact that one of the incidents could well be an act of terrorism or other deadly incident.
Being prepared, observant is this day and time is a must for all citizens in all walks of life. Situational awareness saves lives. Taking the precaution of a lock down at any level of public facility is the minimal acceptable effort to control and minimize possible loss of life of those present in the moment.