(NewsNation) — Several U.S. Republican senators are concerned about terrorists targeting Americans after an affiliate of the Islamic State group claimed responsibility for the Russia concert hall attack that killed more than 130 people Friday.

“I think common sense tells you if they run a trafficking network of people that would most certainly use it to move operatives into the United States,” Sen. Marco Rubio, R-Fla., told ABC’s “This Week.” “I’m not claiming there’s an imminent threat to the U.S., but I am saying that the border situation and the existence of that network is a threat to the United States. If they could do what they did in Moscow in the United States, they would do it in a heartbeat.”

“We have millions of Americans who live or travel abroad throughout Europe and Asia embassy businesses, so I’m deeply worried that ISIS from Afghanistan will end up targeting Americans sooner rather than later,” Sen. Tom Cotton, R-Ark., said on “Fox News Sunday.”

Currently, officials aren’t aware of any specific threat targeting the U.S. However, in testimony this month, a military general and the director of national intelligence warned ISIS poses a significant threat and is assessing potential attacks in the U.S. and Europe. Additionally, two weeks ago, the U.S. Embassy in Moscow issued a warning to avoid large gatherings, including concerts, over extremist threats.

The Moscow attack, for which Islamic State-Khorasan, known as ISIS-K, claimed responsibility, was the deadliest on Russian soil in years.

Military and foreign policy experts in Washington confirmed to NewsNation that ISIS-K has been focused on Russia since about 2014.

The Kremlin launched large-scale attacks against ISIS fighters in Syria to stop their crusade in the Middle East and to support the Syrian president, a longtime ally of Russian President Vladimir Putin.

Investigators charged Dalerdzhon Mirzoyev, 32; Saidakrami Rachabalizoda, 30; Shamsidin Fariduni, 25; and Mukhammadsobir Faizov, 19, with committing a terrorist attack resulting in the death of others. The offense carries a maximum sentence of life imprisonment.

Court statements said two of the suspects accepted their guilt in the assault, though the men’s condition raised questions about whether they were speaking freely. There had been conflicting reports in Russian media outlets that said three or all four men admitted culpability.

The four men, all identified in the media as citizens of Tajikistan, will be held in custody until May 22 pending investigation and trial.

Seven more people were detained on suspicion of involvement in the attack, Putin said in an address to the nation Saturday night. He sought to tie the attack to Ukraine and claimed the assailants were captured while fleeing there. Kyiv has firmly denied involvement.

The hearings came as Russia observed a national day of mourning for the Friday attack on the suburban Crocus City Hall concert venue that killed at least 137 people and injured nearly 200.

The Associated Press contributed to this report.