The man who killed nine people and put nine more in hospital in a college shooting in the US, had built up a cache of more than a dozen weapons.

Investigator Celines Nunez said all the weapons had been purchased legally.

"Six were recovered at the school, seven were recovered at the shooter's residence. All 14 have been traced to a federal firearms dealer."

She said were owned by the shooter, who was a 26 year-old others belonged to members of his family.

The New Zealander who saw four people killed in the shooting now wants to come home.

Jaylen Gerrand was unhurt in the shooting, but his basketball coach Jeff Green said the 20 year-old is obviously distraught at what he saw.

Green said the 20 year-old witnessed the deaths of four people.

"A guy right next to him, he didn't know who it was just got shot in the back of the head and fell to the ground, Jaylen got blood all over him from the head shot, and then he saw a girl get shot and two other guys about forty [or] fifty feet away from him."