May 21 (Reuters) - The U.S. National Transportation

Safety Board said on Tuesday it will send an accredited

representative and four technical advisors to support

Singapore's investigation into a severe turbulence incident on

Singapore Airlines Flight 321.

Under an international aviation treaty, the NTSB can take

part as a representative since the plane was built by Boeing ( BA )

. The investigation will be led by the Singapore Transport

Safety Investigation Bureau.