Russians to Testify in Boston Bombing Trial Under 'Hotel Arrest' Officials: One witness sent home after arriving over security concerns.

BOSTON  -- Several Russian nationals summoned to the U.S. to testify in defense of Boston Marathon bomber Dzhokhar Tsarnaev are staying in the Boston area under "hotel arrest" and are closely monitored by the FBI, law enforcement officials told ABC News.

One of the witnesses called by the defense flew to the U.S. only to be turned around in Boston's airport by American customs agents after he admitted to fighting along Islamist militants in southern Russia, officials said.

“He stated that he fought with rebels in the forests and was on a watch list. Customs put him on a plane back to Russia,” said one federal official with knowledge of the case. A state official at Logan Airport also confirmed that a member of the Tsarnaev party was sent back to his homeland for ties to Muslim extremists.

The remaining Tsarnaev witnesses remain in the Boston area fitted with ankle bracelets and monitored around-the-clock by FBI agents. The defense witness list is under court seal. Five of the witnesses were slated to fly home today, according to court records, but the Tsarnaev trial was suspended until Monday after a juror fell ill.

The Russians were permitted to travel to the U.S. on S-visas, or Significant Public Benefit Parole, issued to aliens considered a security risk who were subpoenaed to testify. Security for aliens here on temporary S-visas is determined by officials from the Department of Homeland Security, who, in the case of Tsarnaev, a convicted terrorist, felt the “strictest of security conditions should apply,” said another federal official with knowledge of the case.

Dzhokhar Tsarnaev’s mother is a native of Dagestan, and now lives there with her ex-husband Anzor. The couple returned to the Russia from the U.S. in 2012, the same year that Dzhokhar's older brother, Tamerlan Tsarnaev, spent nearly seven months there, arriving in January and flying home in July. While there, he reportedly attempted to connect with Islamist rebels.

Since the outset of Dzhokhar's trial, the defense has attempted to paint Tamerlan, who was killed in a police shootout three days after the marathon blasts, as the extremist behind the plot, with Dzhokhar just following his older brother's lead.

This week Assistant U.S. Attorney William Weinreb complained to a federal judge that the Russian witnesses' trip to the U.S. -- which is being funded by federal taxpayers as part of the 21-year-old bomber’s defense team’s strategy -- has become an “enormous expense and distraction” for the FBI.

“The FBI is devoting 16 personnel full time to taking care of them, both guarding them as well as protecting them from the press and from others,” Weinreb told the judge. “It’s an enormous expense and distraction from the agency, and that’s just part of the expense that the government has endured.”

The federal government also paid for the flights of the family members from Russia -- including the unnamed witness who was flown back to Dagestan after he admitted to terrorist ties. A day after their arrival, the FBI was forced to relocate the Russians to another hotel last week after the Hampton Inn in Revere near Boston’s Logan Airport was besieged by reporters.

FBI spokeswoman Kristen Setera refused comment citing the ongoing trial. The U.S. Attorney’s Office also declined to comment. The law enforcement officials spoke on condition of anonymity because they are not authorized to discuss the case. Tsarnaev’s defense team did not respond to requests for comment.

Another defense witness, Magomed Dolokov, a physicist from the Russian Caucasus who planned to attend a master's program at MIT, was interviewed multiple times by the FBI after the bombings but has since vanished, Tsarnaev defense attorney Miriam Conrad told a judge this week.

Dolokov told the FBI he met Tamerlan Tsarnaev at the Prospect Street mosque in Cambridge in July 2012 and was with both brothers three days before the blasts, a portion of that day was captured on a video at the Wai Kru Gym which was entered into evidence by the defense.

Dolokov told the FBI he met Tamerlan’s wife, Katherine Russell, at their Cambridge home in August 2012, the same night that “Tamerlan told Dolokov he was going to join the mujahedeen,” in Dagestan, according to one of the FBI reports.

On April 12, 2013, three days before the Boston Marathon blasts, Dolokov spent the day with both brothers and Tsarnaev’s defense suggested a video of the three Russians in a Wai Kru boxing ring was evidence that Tamerlan bullied Dzhokhar, pointing to a segment where the older brother threw hand wraps at the then 19-year-old as he leaned passively against the ropes.

Dzhokhar Tsarnaev was convicted in early April of all charges contained in a 30-count federal indictment related to the marathon bombing, which killed three people and injured some 260 others.