China’s naval buildup now includes a new class of attack submarine that employs both conventional and nuclear propulsion, according to defense officials.

The sinking of China’s first Zhou-class submarine represents a setback for Beijing as it continues to build out the world’s largest navy.

In May 1969, the USS Guitarro, a Sturgeon-class nuclear-powered submarine, sank while under construction at the Mare Island Naval Shipyard due to human error. -Two teams of civilian engineers were working on the sub’s ballast tanks without coordinating,