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JOINT BASE ANDREWS, Md. — President Donald Trump was paying tribute Saturday to the four Americans killed in a suicide bomb attack in Syria this week as he set off to Dover Air Force Base for the return of their remains.

The trip was not listed on the president’s public schedule that was released Friday night, but he tweeted the news before his Saturday morning departure from the White House.

“Will be leaving for Dover to be with the families of 4 very special people who lost their lives in service to our Country!” he wrote.

The Islamic State group has claimed responsibility for Wednesday’s attack in the northern Syrian town of Manbij that came about a month after Trump had declared that the militants had been defeated and that he was withdrawing U.S. forces from the country.

The attack highlighted the threat still posed by IS despite Trump’s assertion and could complicate that withdrawal plan. Some of his senior advisers have disagreed with the decision and have offered an evolving timetable for the removal of the approximately 2,000 U.S. troops.

The bombing, which also wounded three U.S. troops, was the deadliest assault on U.S. forces in Syria since they went into the country in 2015.

At least 16 people were killed, and the dead were said to have included a number of fighters with the Syrian Democratic Forces, who have fought alongside the Americans against IS.

The Pentagon has identified three of the four Americans killed:

—Army Chief Warrant Officer 2 Jonathan R. Farmer, 37, of Boynton Beach, Florida, who was based at Fort Campbell, Kentucky.

—Navy Chief Cryptologic Technician (Interpretive) Shannon M. Kent, 35, of Pine Plains, New York, and based at Fort Meade, Maryland.

—civilian Scott A. Wirtz from St. Louis.

The Pentagon hasn’t identified the fourth casualty, a civilian contractor.

Trump has made one other visit to Dover during his presidency, soon after taking office. On Feb. 1, 2017, Trump honored the returning remains of a U.S. Navy SEAL killed in a raid in Yemen. Chief Special Warfare Operator William “Ryan” Owens, a 36-year-old from Peoria, Illinois, was the first known U.S. combat casualty since Trump became president.

Over the past month, Trump and others have appeared to adjust the Syria pullout timeline, and U.S. officials have suggested it will likely take several months to safely withdraw American forces from Syria.

In a Dec. 19 tweet announcing the withdrawal, Trump had said, “We have defeated ISIS in Syria, my only reason for being there during the Trump Presidency.” He said the troops would begin coming home “now.” That plan triggered immediate pushback from military leaders, including the resignation of Defense Secretary Jim Mattis.

Manbij is the main town on the westernmost edge of Syrian territory held by the U.S.-backed Syrian Kurds, running along the border with Turkey. Mixed Kurdish-Arab Syrian forces liberated Manbij from IS in 2016 with help from the U.S.-led coalition.

But Kurdish control of the town infuriated Turkey, which views the main U.S. Kurdish ally, the YPG militia, as “terrorists” linked to Kurdish insurgents on its own soil.

Trump reinforced his withdrawal decision during a meeting with about a half-dozen GOP senators late Wednesday at the White House.

Sen. Rand Paul of Kentucky, who was at the meeting, told reporters on a conference call that the president remained “steadfast” in his decision not to stay in Syria – or Afghanistan – “forever.” But the senator did not disclose the latest thinking on the withdrawal timeline.

Paul said Trump told the group, “We’re not going to continue the way we’ve done it.”