WASHINGTON -- Two U.S. officials traveling with diplomatic passports were drugged while attending a conference in Russia last year, and one of them was hospitalized, in what officials have concluded was part of a wider, escalating pattern of harassment of U.S. diplomats by Russia.

The incident at a hotel bar during a UN anticorruption conference in St. Petersburg in November 2015 caused concern in the U.S. State Department, which quietly protested to Moscow, according to a U.S. government official with direct knowledge of what occurred.

But it wasn't until a dramatic event in June, when an accredited U.S. diplomat was tackled outside the U.S. Embassy in Moscow, that officials in Washington reexamined the November drugging and concluded they were part of a definite pattern.

The State Department suggested the harassment has become a particular concern in the past two years.

According to the U.S. government official, and another former official also knowledgeable about the case, the drugged diplomats were part of a delegation of Americans attending the Conference of the States Parties to the United Nations Convention against Corruption, held on November 2-6 in St. Petersburg.

It was the first official conference in Russia that U.S. government representatives were allowed to travel to since the United States, the European Union, and their allies imposed sanctions on Russia for its annexation of Ukraine's Crimean Peninsula in 2014.

A conference list shows scores of attendees from around the world, including 21 people from various bureaus within the U.S. State and Justice departments. Other Americans were also in attendance, including academics and representatives of nongovernmental organizations.

'Date Rape Drug'

RFE/RL contacted several of the people attending from nongovernmental organizations; all said they were unaware of the drugging.

The U.S. government official told RFE/RL that U.S. investigators concluded that the two Americans -- a man and a woman -- were slipped a so-called date rape drug, most likely at a bar in the St. Petersburg hotel where they were staying.

One of the Americans was incapacitated and brought to a Western medical clinic in the city for treatment, and to have blood and tissue samples taken in order to determine precisely what caused the sudden illness. However, while the person was at the clinic, the electricity suddenly went out and the staff was unable to obtain the necessary tissue samples, the official said.

The individual was then flown out of the country for further medical treatment, but by then it was too late to gather proper samples, the official said.

Because the U.S. officials in attendance at the conference were not top-level State or Justice officials, the State Department decided to take a quiet approach to the incident.A formal note of protest was lodged, the official said, but Russian authorities asked for evidence that the person had been drugged, and the Americans lacked samples.

When investigators sought timesheet records for personnel working at the hotel where the U.S. officials had been staying, the hotel managers said there were none for that particular period, the official added -- a claim that also raised suspicions.

In the end, the U.S. government official said, the response given by Russian officials to the investigators looking into the drugging was: Without more evidence, there's nothing more we can do.

Unnerving Incidents

The Russian Embassy did not respond to requests for comment from RFE/RL.

Incidents of foreigners being drugged in Russian bars are not uncommon and are often linked to robbery attempts. The State Department has a specific advisory regarding the danger of unattended drinks at Russian bars.

But the account of the drugging given by the U.S. official indicates that they did not resemble the typical scenario in such incidents, which often unfold in crowded bars and nightclubs. The two individuals were drugged separately, and are believed to have been targeted in the bar of the upscale St. Petersburg hotel where they were staying, the official said. There were no indications that they were victims of attempted robbery.

The issue of U.S. diplomats and journalists being harassed by Russian and Soviet intelligence agencies dates back decades, though with varying degrees of seriousness. U.S. officials have reported minor, if unnerving, incidents involving residences in Moscow and elsewhere being broken into and household items moved around, a gas stove left on, or a cigarette left burning.

In recent years, however, there has been a noticeable uptick in reports of harassment, even before U.S.-Russian relations spiraled downward over Moscow's interference in Ukraine.

A 2013 report by the State Department's Inspector General said "employees face intensified pressure by the Russian security services at a level not seen since the days of the Cold War."

Michael McFaul, the U.S. ambassador from 2012 until February 2014, was on several occasions accosted by crews from state-controlled television channels who showed up outside his private meetings without prior notification, prompting Washington to complain to Moscow about security concerns. McFaul suggested his communications were being tapped and leaked to the journalists.

Problem 'Escalating'

The spokesman for the U.S. Embassy in Moscow said earlier this year that water faucets had been discovered mysteriously left running in apartments in the past. And on at least two occasions over the years, U.S. officials have said diplomats have found human excrement on the floor of apartments.

But almost all of the known complaints involving diplomats have been about harassment rather than physical harm.

Asked for official comment about the drugging, the State Department suggested that the problem has escalated since 2014.

"Without speaking to specific incidents, we are troubled by the way our employees have been treated over the past two years," a State Department official, speaking on condition of anonymity, told RFE/RL. "We have raised, and will continue to raise, at the highest levels any incidents inconsistent with protections guaranteed by international law."

In June, the issue gained new attention when an American entering the U.S. Embassy in Moscow was tackled by a Russian guard. The American was identified by the State Department as an accredited diplomat, and said he had shown his identification to the guard under normal procedure.

Russian Foreign Ministry officials, however, said the guard, who was employed by the country's main security agency, the FSB, was only doing his duty: protecting the embassy from what he deemed to be a suspicious person who, they said, was also wearing a disguise.

Tit-For-Tat Expulsions

Later, Moscow claimed the American was a CIA officer working under diplomatic cover, a common technique used by many countries, including both Russia and the United States.

The incident was captured on video, and the footage was later aired on Russian television.

In the aftermath, Moscow and Washington each kicked out two of the other side's accredited diplomats in tit-for-tat expulsions that were reminiscent of the Cold War.

U.S. officials have also said that in the weeks before the embassy incident, American diplomats were pulled over by Moscow traffic police dozens of times, which is unusual in a city where diplomats are usually afforded leeway for minor traffic violations.

In June, amid the U.S. outcry over the treatment of its diplomats in Russia, Moscow complained that pressure tactics were being used against its diplomats in the United States. Foreign Ministry spokeswoman Maria Zakharova suggested that Russia was simply retaliating -- and warned that it could get worse.

"Our diplomats are constantly coming up against provocations from the FBI and the CIA, who conduct unacceptable measures against them, including psychological pressure in the presence of their families," Zakharova told a briefing in Moscow on June 28.

She added that "diplomacy is based on reciprocity. The more the U.S. damages relations, the harder it will be for U.S. diplomats to work in Russia."

