*sigh*
Small caliber semi-auto handgun.
Probably from the stands to his right rear (viewers left). We'll have to wait for video from or of that location (and there will be at least some) to confirm what happened to the flurry of follow up rounds that were fired. It's possible those were fired as the shooter was being tackled by bystanders.
Not a professional.
*sigh**sigh*
It was a rifle.
From a rooftop just outside of the event.
Witness saw the shooter on the roof before he fired. Told police and Secret Service. They did not arrive at the scene in time to prevent the first shot.
The flurry of follow up shots was law enforcement killing the shooter.
The reason why it sounded like small caliber fire is the distance from the recording microphones.
Secret Service counter sniper team.
They had a general direction to look (from the civilian warnings a minute or two earlier).
They did not have sight of the shooter or know exactly which roof the assassin was on until the assassin fired.
At least one of the counter snipers returned fire almost instantly. But did not kill the assassin with his first shot.
This was NOT the only SS counter sniper team on site. When the assassin continued firing, both (or more) teams took him out.
...