DIYARBAKIR, Turkey — Islamic State on Thursday claimed it had killed a 61-year-old American citizen working at a Turkish air base used by U.S. warplanes to stage assaults on the militant group in neighboring Syria and Iraq.

Thaddeus Borowicz’s body was discovered Monday morning at the entrance to his apartment building in the southeastern city of Adana, nearly 65 miles from the Syrian frontier. Initial news reports said that the Traverse City, Mich., native had fallen from the 10th floor after locking himself out of his unit and attempting to enter through a window.

His death came to light after he failed to turn up for a shift at Incirlik Air Base, according to reports.

Borowicz was a contractor with Vectrus, a firm based in Colorado Springs, Colo., and was assistant chief of the fire department at Incirlik Air Base. He had worked there about 20 years.

He was found dead by co-workers near his apartment, according to George Rhynedance, a Vectrus spokesman.

Lt. Col. David Westover, a military spokesman for U.S. European Command, which oversees operations at Incirlik, said the U.S. military had “no reports of any Americans killed” in Adana by Islamic State.

“However, there was a tragic accidental death that occurred earlier this week, when a U.S. contract employee died at his home,” he said from Stuttgart, Germany.

The Islamic State-affiliated Amaq News Agency, however, quoted a security source saying that it had assassinated “an American officer” at his home on Monday and that the claim of responsibility had been delayed to “ensure the safety” of the team sent to execute Borowicz.

Rita Katz, director of the private SITE Intelligence Group, noted on social media that the killing was likely the second Islamic State attack in Turkey over the last week.

On Sunday, two Islamic State operatives wielding pistols attempted to assassinate the Syrian journalist Ahmed Abd al-Qader in the ancient, southeastern city of Sanliurfa. That city, along with nearby Gaziantep and Kilis, is a hub of militant Islamist activity, primarily for logistics, recruitment and resupply purposes.

Abd al-Qader, who edits the online publication Eye on the Homeland, survived a similar assassination attempt earlier this year. He is reportedly in a stable situation in a Turkish hospital.

U.S. fighter jets have used the strategic Incirlik base, which played a prominent role during the Cold War, since the 1950s, including during the invasions of Afghanistan and Iraq.

Turkey permitted U.S. warplanes to once again fly attack operations from Incirlik last year – greatly reducing flight times for U.S. bombers pounding Islamic State in Syria and Iraq –after months of tense negotiations. The Pentagon in March ordered military family members to leave Incirlik due to a heightened risk of terror attacks.

Turkey’s southern border zones have become a jihadist superhighway over the last five years as extremists flooded Syria to join the insurgency against Syrian President Bashar Assad

Ankara long turned a blind eye to the radical Islamists using its southern cities and towns for logistics and resupply purposes, critics say. A series of suicide bombings and assassinations over the last year has forced Ankara to recalibrate and take a more forceful approach to extremism.

However, most analysts note that Islamic State — as well as a profusion of other extremist groups — has embedded itself in major metropolitan centers throughout the country.

Meantime, Belgian intelligence reportedly warned its police Wednesday that a number of armed Islamic State operatives were preparing to infiltrate Europe via Turkey’s Aegean smuggling routes.

‘Their action is imminent,” a memo issued to the security services said, according to Agence France-Presse.