(CNN) More than a year after migrants in the custody of US Customs and Border Protection began to die, the agency released a plan on Tuesday to improve medical screening.

Doctors said they were disappointed it took that long to come up with the plan, which is lacking in many details and provides health screenings only for children, not adults.

"To me, this is beyond disappointing. It's incredibly frustrating," said Dr. Joshua Sharfstein , a professor at the Johns Hopkins Bloomberg School of Public Health.

"This agency is responsible for people's lives and should act like it is," he added.

Four children died in CBP custody from December 2018 through May 2019. Three died of the flu and one died of sepsis. Adults in CBP custody have also died in the past year.