The powerful intercontinental missile tested by North Korea late last year is “highly likely” to have been built with foreign blueprints or parts, according to a new technical analysis that describes multiple similarities between Pyongyang’s new missile and ones built by the Soviet Union decades ago.

The foreign assistance — the precise nature of which is still unclear — could explain why North Korea apparently was able to skip the months and even years of preliminary testing normally associated with any advanced new missile system, the report by U.S. and German experts says.

The missile dubbed Hwasong-15 had never been seen publicly until its successful maiden test on Nov. 28, when it flew 2,780 miles above the Earth in a nearly vertical trajectory before splashing into the Sea of Japan, also known as the East Sea. The 75-foot-tall colossus was one of two intercontinental ballistic missiles to appear abruptly on North Korean launchpads last year and the first with sufficient range to strike cities across the entire continental United States.

Intelligence agencies have long believed that North Korea incorporated Soviet designs in many of its missiles, including a submarine-launched ballistic missile successfully tested in 2016. But experts have been mystified over North Korean leader Kim Jong Un’s rapid gains in long-range missile technology, including back-to-back successful tests of two different ICBMS last year.

The new report builds an elaborate, if partly circumstantial, case linking North Korea’s newest missile to Soviet designs dating as far back as the mid-1960s. The evidence includes striking similarities between the Hwasong-15 and a family of Soviet-era missiles, including one that was developed by Russian engineers but abandoned before production began, according to the report prepared for Jane’s Intelligence Review, a British-based journal that focuses on international security threats. A draft of the report was provided to the Washington Post.

“It is highly likely that North Korea made use of external knowledge, technology, or hardware, in the development of the Hwasong-15 ICBM,” states the report, authored by Markus Schiller, a Munich-based space technology analyst, and Nick Hansen, an imagery specialist with a 47-year career with the U.S. intelligence community.

Based on new computer modeling and enhanced images of the North Korean missile, the researchers concluded that the foreign support “was derived from the Soviet-era ballistic missile program,” though it is unclear exactly when or how the transfer took place, the report says.

The researchers found, for example, that the North Korean missile’s size and shape echo those of the UR-100, a two-stage solid-fuel missile built by the Soviet Union in the 1960s and 1970s, with a few differences. Its engine shares the same distinctive dual-nozzle configuration as the Soviet-made RD-250 missile engine first built in 1965 and appears to use the same potent fuel mixture — a high-energy liquid propellent that only recently came into use in North Korea.

The similarities appear to implicate the former Soviet Union as the original source of the technology, and not China or Iran, as some analysts have speculated, the researchers said.

While the similarities with the UR-100 are striking, the authors posit that the Hwasong-15 may actually be a clone of a different Soviet-era missile that was never brought into full production. That missile, the R-37, was developed as part of a competition between two rival missile-design bureaus as the Soviet Union searched for an answer to the Minuteman ICBM developed by the United States in the 1960s. The UR-100 won the competition, and the R-37 — which was similar in size and shape and apparently used the RD-250 engine — was canceled.