WASHINGTON — Members of Congress demanded answers Thursday two weeks after an ambush in the African nation of Niger killed four U.S. soldiers, with one top lawmaker even threatening subpoenas. The White House defended the slow pace of information, saying an investigation would eventually offer clarity about a tragedy that has morphed into a political dispute in the United States.

Among the unresolved inquiries: Why were the Americans apparently caught by surprise? Why did it take two additional days to recover one of the four bodies after the shooting stopped? Was the Islamic State responsible?

The confusion over what happened in a remote corner of Niger, where few Americans travel, has increasingly dogged President Donald Trump, who was silent about the deaths for more than a week.

Asked why, Trump on Monday turned the topic into a political tussle by crediting himself with doing more to honor the dead and console their families than any of his predecessors. His subsequent boast that he reaches out personally to all families of the fallen was contradicted by interviews with family members, some of whom had not heard from Trump at all.

And then the aunt of an Army sergeant killed in Niger, who raised the soldier as her son, said Wednesday that Trump had shown “disrespect” to the soldier’s loved ones as he telephoned to extend condolences while they were driving to the Miami airport to receive his body. Sgt. La David Johnson was one of the four Americans killed Oct. 4.

In an extraordinary White House briefing, John Kelly, the former Marine general who is Trump’s chief of staff, described himself as “stunned” and “brokenhearted” by the criticism of Trump. He also invoked his son serving in Iraq to explain why American soldiers operate in dangerous parts of the world, saying their efforts to train local forces mean the U.S. doesn’t have to undertake large-scale invasions of its own. Kelly’s other son, Robert, was killed in combat in Afghanistan seven years ago.

The deadly ambush in Niger occurred as Islamic militants on motorcycles, toting rocket-propelled grenades and heavy machine guns, attacked a U.S. convoy. Two other Americans were wounded.