The source said the assessment was conducted about a month ago and includes satellite imagery of a concrete launch facility and other constructions, including bunkers, the US media reported.
The source also indicated that the work at the site "fits the pattern" for Russia's S-400 system, the report said.
The report gave no indication about where the site is located in Turkey.
US officials have warned that if Ankara goes ahead with plans to purchase the Russian S-400 anti-missile systems, the United States could withhold the transfer of F-35 jets. The US Congress also recently passed legislation that would block the transfer of F-35s if Turkey buys the Russian S-400s.
Last week, Turkish Prime Minister Recep Tayyip Erdogan confirmed that Turkey expects to receive the Russian S-400 missile defense systems in the near future.