Several lawmakers signed a letter Friday seeking to have the Department of Homeland Security and Customs and Border Protection open an investigation into the death of an 8-year-old Guatemalan boy while he was in U.S. custody.

Felipe Gomez Alonzo is the second child to die in those circumstances this month, following the earlier death of 7-year-old Jakelin Caal.

The letter was shared on Twitter by Sen. Tom Udall, D-N.M. It was co-signed by Sens. Martin Heinrich, D-N.M.; Kamala Harris, D-Calif.; and Kirsten Gillibrand, D-N.Y.; as well as Rep. Ben Ray Lujan, D-N.M.

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The document, which was addressed to officials including Homeland Security Secretary Kirstjen Nielsen, said the boy’s death was “unacceptable.”

“We seek immediate answers from the Department of Homeland Security (DHS) and CBP about how they will ensure the health and safety of children at our borders, adequately train CBP agents and officers and allocate resources, and prevent such tragedies from happening in the future,” the letter said.

The boy and his father arrived in the U.S. at El Paso, Texas, on Dec. 18 and were taken to the Border Patrol’s Alamogordo station several days later, according to Guatemala’s foreign ministry.

An agent first “noticed that the child showed signs of potential illness” on Monday and brought the boy and his father to a medical center in Alamogordo, New Mexico, where the boy received a diagnosis of a “common cold,” immigration officials previously said.

GUATEMALAN BOY, 8, DIES IN US BORDER PATROL CUSTODY

Prior to the child's leaving the hospital, though, staffers detected a fever. They discharged the boy in the midafternoon with prescriptions for amoxicillin and Ibuprofen.

Later in the evening, the child “exhibited nausea and vomiting and was transferred back” to the facility for evaluation and treatment, CBP said. He died around midnight.

An autopsy performed on the boy earlier this week showed that he had influenza, New Mexico authorities said on Thursday. However, more tests needed to be done before a cause of death could be determined, the New Mexico Office of the Medical Investigator said in a statement.

The lawmakers' letter noted that following his discharge, the boy was brought to a temporary holding facility, citing a timeline from CBP. Such facilities “are not designed for overnight custody, yet CBP routinely uses these facilities in this way, and has been detaining people for prolonged periods of time because of this Administration’s policies,” the document said.

BOY WHO DIED IN US CUSTODY HAD INFLUENZA, AUTOPSY CONFIRMS; DHS BOSS NIELSEN PLANS BORDER-REGION VISIT

The letter also acknowledged Nielsen’s pledge for all children in the future to receive a “more thorough” medical screening following apprehension.

“It is well past the time for CBP to re-evaluate its policies and procedures for the care and treatment of children,” the letter said.

The lawmakers gave a Jan. 15 deadline for responses to a number of requests, including “complete accounting and documentation from DHS, CBP and the DHS Office of Inspector General about their investigation into Felipe’s death,” as well as a briefing on what they discovered.

Fox News’ Greg Norman, Amy Lieu and Frank Miles contributed to this report.