Multiple reports say that the launch of a separate missile from that vehicle was unprecedented.
The Financial Times and Wall Street Journal reported that the Pentagon was surprised by the test, which showed that China had advanced its nuclear weapons.
The test, thought to have taken place in July, made global headlines when it was revealed that the vehicle had flown into space and completed a partial circle of the Earth.
A separate missile that fell into the South China Sea is believed to have been part of the test. The ability to launch a missile from a parent vehicle at five times the speed of sound has not been demonstrated by the United States or Russia.
The Financial Times reported that US military experts are trying to understand how China mastered the technology, which puts it ahead in the hypersonic arms race. The report was confirmed by the Wall Street Journal.
The Pentagon and US intelligence were alarmed by the Chinese test. In it, the launch vehicle, a long-range missile, carried the guided hypersonic warhead around the world and then released it toward a test target inside China.
A senior Pentagon official said last week that the hypersonic missile missed its target by more than 20 miles, which was close enough.
The role of the missile was not clear. It could be used to target an enemy country.
It underscored the speed with which China has mounted a strategic challenge to the US military.
The Pentagon says China has accelerated the production of nuclear warheads, which could be carried by a hypersonic missile as well as conventional submarine and land-based missiles.
The July hypersonic test and a second one three weeks later should create a sense of urgentness in the US, according to CBS News.
