The Dutch intelligence service says it prevented a Russian spy from infiltrating the International Criminal Court.

It has all the elements of an espionage thriller: An accused Russian spy who concocted an identity as a Brazilian. The creation of an elaborate cover story. And what Dutch authorities said appeared to be a foiled plot to gain access to the International Criminal Court as it investigates Russian war crimes.

Those details emerged this week in a real-life case in which Dutch officials said 36-year-old Sergey Vladimirovich Cherkasov spent years building an identity as a Brazilian citizen, polishing a résumé that got him an internship at the International Criminal Court in The Hague before Dutch officials blew his cover.

According to Dutch intelligence, Mr. Cherkasov pretended to be a Brazilian named Viktor Muller Ferreira, and got an internship at the court using a detailed cover story that hid his ties to the Russian military intelligence agency, the G.R.U.

Mr. Cherkasov was due to start working at the court, but was denied entry to the Netherlands at Amsterdam’s Schiphol Airport in April after the AIVD, the Dutch intelligence agency, tipped off immigration officials. He was sent back to Brazil and declared an “undesirable alien,” intelligence officials said in a statement Thursday. Officials did not say how they identified him as a spy.

The International Criminal Court is investigating potential war crimes by Russia in its invasion of Ukraine, as well as the Russian-Georgian war in 2008.

“If this person had gotten the chance to really work at the I.C.C., he could have gathered information, could have spotted sources (or recruited them) and could have gained access to the digital systems,” the Dutch intelligence service said in a statement. The G.R.U. has been blamed for cyberattacks on the U.S. and Ukraine.

The Dutch intelligence agency published a document that it said Mr. Cherkasov probably wrote in 2010, laying out a cover story that included specific details about the purported background of Mr. Ferreira, including which high school he attended and how many students were at the school; health information about his aunt; a crush he had on a teacher; and how much rent he paid for an apartment in Brasilia.

It can be hard to know what is true from such cover stories, because they are often a mixture of true and false information, including personal observations that are hard to disprove, Dutch intelligence said.

The document blacks out the names of institutions and other details, though he appeared to have a connection to Johns Hopkins University.

Eugene Finkel, a Ukraine-born associate professor at Johns Hopkins University, wrote on Twitter that Mr. Cherkasov had been in his class and that he had written him a recommendation letter: “A strong one, in fact. Yes, me. I wrote a reference letter for a GRU officer. I will never get over this fact.” Mr. Finkel declined a request for comment.

Johns Hopkins, in a statement, said it had “confirmed that a person of this name graduated from our institution in 2020” and was monitoring developments but had “no further information to share at this time.”

The International Criminal Court said that it was “very thankful to the Netherlands for this important operation and more generally for exposing security threats.”