A group of doctors from Harvard and Johns Hopkins has urged Congress to investigate the deaths of six migrant children who were held in government custody after crossing the southern border in the past year, warning that “poor conditions” at U.S. facilities are increasing the risk of spreading deadly infectious diseases, especially the flu.

The doctors, who wrote to Congress on Thursday, said 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. Child flu deaths are rare, the doctors said, and should be preventable.

“Poor conditions at the facilities may be amplifying the spread of influenza and other infectious diseases, increasing health risks to children,” said the letter submitted by Harvard pediatrics professor Jonathan Winickoff; Johns Hopkins public health professors Joshua Sharfstein and Paul Siegel and two of their master’s students; and San Francisco forensic pathologist Judy Melinek. “With so many lives at risk, these issues are worthy of congressional investigation. Another influenza season is around the corner, and there are other types of infectious diseases that pose a threat to detained populations. Timely action is critical.”

The letter alleged that the Department of Homeland Security and the Department of Health and Human Services — which supervises longer-term custody of unaccompanied migrant minors — might not be following best practices in regard to screening, treatment, isolation and prevention of the flu.

Sharfstein said in an interview that with all of the problems at the southern border “people may be overlooking the risks of outbreaks that are entirely preventable.”

The concerns come as the U.S. government has been struggling to deal with a record influx of Central American families and unaccompanied minors at the border, a shift in migration patterns that has at times put incredible stress on the nation’s immigration infrastructure. Children have spent days or even weeks in Border Patrol stations that are not meant for long-term housing, and the crush has meant people have been jammed into holding pens and even have had to stay in outdoor fenced encampments without access to showers or beds.

U.S. officials have said they are doing their best to care for everyone they take into custody but have acknowledged that Border Patrol stations are not designed to handle the number of migrants that have been crossing the border.

Detainees with the flu, including children, “are handled as appropriate depending on the specific circumstances,” a Customs and Border Protection (CBP) spokesperson said. Children who test positive for flu are placed together “as best as possible” to separate them from other children, the spokesperson said.

Five Guatemalan children who had been taken into custody died between December and May. At least three of the deaths involved the flu, according to autopsy reports obtained by the Washington Post. A sixth child, a 10-year-old girl from El Salvador with a history of heart problems, died in September 2018 after a long illness, administration officials disclosed in May.

Before the recent deaths, it had been 10 years since a child died in Border Patrol custody, officials have said.

Border Patrol facilities had little to no health care resources before the December deaths of Jakelin Caal Maquin, 7, and Felipe Gomez Alonzo, 8. DHS officials then deployed medical personnel to border stations.“CBP has approximately 200 medical personnel engaged along the Southwest border. This is a significant increase,” an agency spokesperson said.