In the early morning of June 6, a uniformed Russian Federal Security Service (FSB) guard stationed outside the U.S. embassy in Moscow attacked and beat up a U.S. diplomat who was trying to enter the compound, according to four U.S. officials who were briefed on the incident.

This previously unreported attack occurred just steps from the entrance to the U.S. Embassy complex, which is located in the Presnensky District in Moscow’s city center. After being tackled by the FSB guard, the diplomat suffered a broken shoulder, among other injuries. He was eventually able to enter the embassy and was then flown out of Russia to receive urgent medical attention, administration officials confirmed to me. He remains outside of Russia.

The attack caused a diplomatic episode behind the scenes that has not surfaced until now. The State Department in Washington called in Russian Ambassador Sergey I. Kislyak to complain about the incident, an administration official said.

The motive for the attack remains unclear. One U.S. official told me that the diplomat was seeking refuge in the embassy complex to avoid being detained by the Russian intelligence services. A different U.S. official told me the diplomat may have been working as a spy in Russia under what’s known as “diplomatic cover,” which means he was pretending to be a State Department employee.

Spokesmen for the both the State Department and the CIA declined to comment on the incident or whether or not the diplomat was in fact an undercover U.S. spy.

In 2013, Russian intelligence services arrested U.S. diplomat Ryan Fogle, whom they accused of secretly working for the CIA. Fogle, who was working as a third secretary in the political section of the U.S.Embassy in Moscow, was arrested carrying various disguises and other tools of spycraft. Russia accused him of trying to recruit Russian intelligence officers.

After interrogating Fogle, the Russian government released him to U.S. officials, but not before humiliating him in the Russian media and chastising the U.S. government for spying inside Russia. Fogle was forced to leave Russia.

If the U.S. diplomat attacked on June 6 was not a spy, U.S. officials have no other explanation for why the FSB guard was trying to stop him from entering the embassy. FSB guards are stationed outside the U.S. Embassy regularly, administration officials said.

Russian harassment of U.S. diplomats in Russia and several other European countries has increased significantly since U.S. sanctions were levied on Russian officials and President Vladimir Putin’s associates in 2014.

On Tuesday, the Russian foreign ministry’s spokeswoman, Maria Zakharova, responded directly to my column at a press conference and on Twitter and accused the U.S. government of deliberately undermining bilateral ties.

“Diplomacy is based on reciprocity. The more the US damages relations, the harder it will be for US diplomats to work in Russia,” she said.

Either way, the fact that the FSB is willing to attack a U.S. diplomat and beat him up right in front of the American Embassy reflects that the Russian security services are becoming increasingly brash, said Evelyn Farkas, who served as deputy assistant secretary of defense for Russia, Ukraine and Eurasia for the Obama administration.

“If this is true, it’s another example of the Russian security services demonstrating a willingness to break taboos,” she said. “The fact that they are using these brutal tactics against foreigners is taking things to a whole other level.”

(c) 2016, The Washington Post · Josh Rogin