(Bloomberg) -- Boeing Co. has delivered a missile interceptor for the US’s ground-based system three years late after overcoming vexing subcontractor quality problems, congressional auditors said.

The delivery in December, which wasn’t previously announced, followed “production challenges” because “the boost vehicle contractor mishandled a key avionics component and had to build a new one,” the Government Accountability Office said Thursday in its annual report on US ground and sea-based missile defense programs.

Boeing is the prime contractor for the missile defense system and integrator of the hit-to-kill warheads built by Raytheon Technologies Corp. Northrop Grumman Corp. supplies the boost vehicle for what’s now a 44-interceptor system in Alaska and California intended to destroy incoming potential intercontinental ballistic missiles from North Korea or Iran. The delayed interceptor was intended to replace one already deployed.

But after building the new avionics component, “the contractor subsequently experienced quality issues with a vendor who supplied an electronic power conversion part needed to build the avionics component,” GAO said.  “The contractor worked with a new vendor to supply the part, but the first several production lots were rejected due to quality issues.”

The interceptor saga is an example of how the US Missile Defense Agency “perennially struggles to fully achieve its annual acquisition goals and has had to cancel a number of critical efforts due to cost and technical challenges -- a trend the department indicated must not continue given the importance of these systems,” GAO said.

As the Missile Defense Agency enters its third decade of operations the newest challenge is developing and coordinating with other agencies the development of systems to detect and intercept Chinese and Russian hypersonic weapons. 

“These efforts represent technologies that have considerable risks, but MDA has not taken necessary steps to reduce risks and ensure appropriate oversight from the Department of Defense,” the GAO said. 

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