Doctors from Harvard and Johns Hopkins Universities are urging Congress to open an investigation into the deaths of six migrant children taken into U.S. custody in the past year.

In their letter, the doctors say “poor conditions” at crowded holding facilities make it easy for deadly infections diseases — especially the flu — to spread to many unvaccinated children.

The letter, sent Thursday, says “autopsy reports show that at least three of the children — ages 2, 6 and 16 — died in part as a result of having the flu, a far higher incidence of such deaths than across the general population,” reports the Washington Post. “Child flu deaths are rare, the doctors said, and should be preventable.”

One recipient of the letter was Rep. Rosa DeLauro (D-CT), chair of the subcommittee that oversees the Health Department.

This is alarming, but unfortunately not surprising, given the way the Departments of Health and Human Services and Homeland Security are responding to the crisis on the border,” DeLauro said in a statement to CBS News. “Even under the best circumstances, the flu is a dangerous threat to people,” adding that the Trump administration continues to make the border crisis worse.

“Much of the problem stems from the lack of coordination, the lack of basic standards of care and procedures to ensure children are screened for health issues, treated quickly and safely, and get the proper vaccinations,” DeLauro wrote.

Before September of last year, CBS says, “it had been a decade since any children had died in Border Patrol custody, according to government figures. Since then, five Guatemalan children and one child from El Salvador have died either in custody or soon after their release to hospitals.”

In an interview with the Post, Johns Hopkins public health Prof. Joshua Sharfstein said the federal government should have been prepared for health issues related to increasing migration.

The key point is that these problems are created to a certain extent by poor planning,” Sharfstein said.