Flag state accident investigators have begun piecing together how the CF Crystal collided with the Sanchi in the East China Sea, leaving 31 seafarers missing and causing a major oil spill.

The 75,000-dwt bulker CF Crystal (built 2011) was registered in Hong Kong and the 164,000-dwt tanker Sanchi (built 2008) in Panama.

The clearest evidence of whether either ship deviated from collision avoidance regulations (Colregs) will come from detailed data on the vessels' course recorded in each ship's voyage data recorder (VDR).