People taken into custody in illegal immigration cases sit in a cage at a facility in McAllen, Tex., on June 17. (U.S. Customs and Border Protection's Rio Grande Valley Sector via AP)

WHEN THE head of U.S. Customs and Border Protection appeared at a Washington forum the other day to discuss his agency — which was instrumental in the Trump administration’s family separation debacle — the toughest moments he faced came from hecklers, one of whom shouted, “You’re orphaning children! You’re kidnapping children!” Setting aside those interruptions, Commissioner Kevin K. McAleenan fielded one gentle question after another, never challenged to explain CBP’s role in what a federal judge has characterized as a breakdown in coordination among government agencies that has left hundreds of migrant children stripped from their parents.

It’s not fair to blame the Bipartisan Policy Center, which convened the forum last week with Mr. McAleenan; it’s not that group’s mission to hold administration officials accountable. That job falls to Congress, which has largely failed to do it — and is failing now to address the chaotic bungling that resulted in families being separated with no effective system to reliably reunite them.

The statistical fallout from that fiasco is roughly 700 children, about a third of those who were removed from their parents this spring, who remain even now in the custody of the U.S. government rather than with their families. In most of those cases, the parents were deported before they could be reunited with their children. It could be months or years before they are rejoined; some may never be.

Those stark numbers are just that: numbers. Behind each one is a terrified, traumatized, tearful child — blameless in the mess created by President Trump, Attorney General Jeff Sessions, Homeland Security Secretary Kirstjen Nielsen and lower-ranking agency chiefs such as Mr. McAleenan. As The Post’s Nick Miroff, Amy Goldstein and Maria Sacchetti reported, Border Patrol agents who removed children from their parents provided files to the Department of Health and Human Services, in whose custody children were entrusted, that were “a mess.” Some of the children’s files included their parents’ names and government-assigned numbers; others didn’t. Often the files made no mention of where the parents had been sent.

At HHS, whose Office of Refugee Resettlement bears direct responsibility for migrant children, the agency database includes no means to indicate which of them might have been snatched from their parents. It was a classic case of estrangement between government’s left and right hands — and you’d think Congress, charged by the Constitution with the obligation of oversight, might want to get to the bottom of it.

But Congress under Republican control has been lily-livered when it comes to cracking the whip on Mr. Trump and his lieutenants — witness the cordial reception proffered by most House Republicans to Scott Pruitt, the ethically obtuse now-former administrator of the Environmental Protection Agency, when he appeared before a congressional committee in the spring.

In the case of family separations, the hecklers who berated Mr. McAleenan were out of line, but their questions — specifically, about whether the U.S. government had made orphans of hundreds of children — were not. That’s among the questions a Congress fulfilling its constitutional role would be asking: how it happened, why it happened and who is responsible for one of the most disgraceful and inept acts of governance in recent memory.