Members of the new Congress aim to unravel exactly what went on inside the executive branch in 2018 when undocumented migrant families were separated at the U.S. border with Mexico.

The House Energy and Commerce Subcommittee on Oversight and Investigations held its first hearing Thursday. The event lasted most of the day as groups of people testified under oath.

The first witnesses included members of Congress’s independent watchdog, the Government Accountability Office, as well as the assistant inspector general who recently said the total number of separated children is unknown.

But most questions from lawmakers went to Commander Jonathan White, who oversaw court-ordered family reunifications last year for the U.S. Department of Health and Human Services. White said he didn’t know ahead of time about the zero-tolerance policy that led to thousands of separations, and he never would have backed it.     

“The consequences of separation for many children will be lifelong,” White told the subcommittee.

Migrant families are still separated if authorities decide a parent is a danger to their kids. White said lawmakers need to work on setting more specific standards for when separations should happen.