A U.S. Special Forces soldier who was killed in Afghanistan on Monday came under small-arms fire while on a foot patrol, U.S. military officials said Wednesday.

Army Sgt. 1st Class Mihail Golin, 34, was killed in Nangarhar province’s Achin district, an area in which U.S. forces have been combating both the Islamic State and the Taliban. The Islamic State in Khorasan, the militant group’s Afghan affiliate, established roots there in the mountainous area along Pakistan’s border in 2014, and the U.S. military launched raids and airstrikes there in 2017 as it stepped up its campaign against the group.

U.S. military officials first acknowledged the fatality Tuesday in a news release, adding that four other service members were wounded in the incident. Two of them were receiving treatment at a medical facility and in stable condition, and the other two have since returned to duty, the news release said.

“We are deeply saddened by the loss of one of our own,” said Army Gen. John W. Nicholson Jr., the senior U.S. military commander in Afghanistan. “At this very difficult time our heartfelt sympathies go out to the families and friends of our fallen and wounded brothers.”

Golin was a Special Forces weapons sergeant with 10th Special Forces Group, of Fort Bragg, N.C., the U.S. Army Special Operations Command said in a news release Wednesday. A native of Riga, Latvia, he moved to Fort Lee, N.J., in October 2004 and joined the Army a few months later in January 2005.

Golin initially served as an infantryman, deploying once to Iraq and twice to Afghanistan. He joined Special Forces in 2014, and deployed to Afghanistan in September.

U.S. military officials in Afghanistan have declined to release additional information about the incident. It is currently under investigation, the Pentagon said in a news release.

Navy Capt. Tom Gresback, a U.S. military spokesman in Kabul, said Tuesday that “there has been a tremendous amount of kinetic activity in the Achin district” of late, using a military term that typically connotes firefights and other violence.

“We continue to work very closely with our Afghan partners in aggressively clearing significant amount of territory in southern Nangarhar,” Gresback said in an email. “We continue to apply increased military pressure on ISIS-K, forcing their movement from their front-line positions.”

Gresback said U.S. and Afghan forces are making “significant progress” in the area, and predicted a long winter for militants in the region.

At least 15 U.S. service members died in Afghanistan in 2017, including 11 in hostile actions. Seven of the 11 killed in combat died in Nangahar province. The most recent combat fatality there occurred Aug. 16, when Army Staff Sgt. Aaron R. Butler, 27, was killed by an improvised device. He was a member of the 19th Special Forces Group.

Nicholson told Pentagon reporters in a news briefing Nov. 28 that ISIS-K at one point had a presence in nine districts spanning three provinces in eastern Afghanistan, but that U.S.-led counterterrorism operations had reduced that to three. Militants in Achin were pushed into mountains that border Pakistan and were attempting to move west from there through access points in the mountains, the general said.

“Remember most of these Daesh fighters came from Pakistan,” Nicholson said, using an alternate name for ISIS. “They go through the passes of southern Nangahar and they move back to their home agency.”

The general said that there were likely about 600 to 800 ISIS fighters in Nangahar. An additional 300 were spread out between two other provinces, in his estimation: Kunar province, which borders Nangahar, and Jowzjan, in northwestern Afghanistan.

Golin’s name was initially released Tuesday by the office of Sen. Cory Booker (D.-N.J.).

“Sgt. First Class Mihail Golin served our nation with courage and distinction, and his death is a loss that will be felt across New Jersey,” Booker said. “Let us honor Sgt. Golin’s extraordinary courage by reflecting on his commitment to the nation he loved, and by recognizing the profound debt of gratitude we owe to him and to his family for their sacrifice.”

This story was originally published at 12:49 p.m. on Tuesday and updated with additional information several times.