MOUNT VERNON, Wash. (AP) - Investigation into the cause of the Skagit River bridge collapse has moved underwater. The National Transportation Safety Board is focusing its investigation on certain pieces of the bridge beams. NTSB officials say they are particularly interested in beam "U4," the second crossbeam in the southbound direction, which wound up underwater. The extraction must be slowly executed, to avoid damaging evidence. After the NTSB is satisfied, then the rest of the debris can be removed, making way for the building of a temporary span to replace the portion of the bridge that collapsed. State officials have said the temporary section could be in place, and the bridge open, by mid-June.