Why use "and" before "which"

One group of five planes, a flight of Navy TBM Avengers, on a mission from the Fort Lauderdale Naval Air Station, on December 5, 1945, were the object, along with the Martin Mariner sent to rescue them, and which also disappeared, of one of the most intensive ground sea rescue operations ever conducted, although no life rafts, oil slicks, or wreckage was ever located.