WASHINGTON (Reuters) - The suspect in the Washington Navy Yard shooting, Aaron Alexis, used a valid pass to gain entry to the base, the FBI said on Tuesday.

"Mr. Alexis had legitimate access to the Navy Yard as a result of his work as a contractor and he utilized a valid pass to gain entry to the building," said Valerie Parlave, assistant director in charge of the FBI's Washington field office.

Authorities believe Alexis entered the base's Building 197 with a shotgun and may have gained access to a handgun inside.

There is no information that he had an AR-15 semi-automatic rifle, Parlave said, adding that the shotgun was bought legally in Virginia.

Authorities said the suspect killed 12 people on Monday before police shot him dead.

(Reporting By Susan Cornwell; editing by Christopher Wilson)