Thirteen weapons have been recovered from the gunman behind the mass shooting at a US community college, and all were purchased legally, officials said on Friday.

"So far we have recovered 13 weapons. Six were recovered at the school, seven recovered at the shooter's residence," said Celinez Nunez, an agent with the US Bureau of Alcohol, Tobacco, Firearms and Explosives.

"All were traced to a federal firearms dealer," she told a news conference a day after the shooting in Roseburg, Oregon that left 10 dead including the shooter.