The U.S. State Department says there is no evidence North Korea was involved in the sinking of a South Korean naval patrol vessel on Friday.

In a meeting with foreign correspondents in Washington on Monday, Deputy Secretary of State James Steinberg said the U.S. has found no reason to believe that a third party was involved in the accident but stressed that a thorough investigation must be conducted before a final determination can be made.

Steinberg also expressed his sympathy to the families of the 46 sailors who remain unaccounted for.

The “Cheonan” sank Friday night following an explosion while patrolling waters about one-point-eight kilometers southwest of Baengnyeong Island near the maritime border with North Korea.