Authorities say Felipe Gomez Alonzo died at a New Mexico hospital after suffering coughing, vomiting and a fever. It was the second such death this month.
On Tuesday, Castro called for a congressional investigation of the death.
At the USA southwestern border with Mexico, there are areas where no medical workers are available.
The father of the 8-year-old boy said he took his son to the border after hearing rumours that parents and their children would be allowed to migrate to United States.
In response to the two migrant children's deaths, Homeland Security Secretary Kirtsjen Nielsen was planning to visit the U.S. -Mexico border region Friday and Saturday, Politico reported, citing a department spokesperson Katie Waldman.
The Trump administration has long argued smugglers capitalize on vulnerable parents because of "loopholes" in American law, such as anti-trafficking legislation passed in 2008 that effectively prevents the immediate deportations of Central American children.
The boy, identified by Guatemalan authorities as Felipe Gómez Alonzo, had been in CBP's custody with his father, Agustin Gomez, since December 18.
The boy was identified as Felipe Alonzo-Gomez, US Rep. Joaquin Castro said.
She said that was a 20 percent increase in rescues from the year before.
Both Felipe and Jakelin came from rural communities with extreme poverty.More news: Authorities close airport as Italy's Mount Etna erupts
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Early in the morning, CBP filed a "placement request" for Felipe and his father to Immigration and Customs Enforcement's (ICE) family placement inbox. "But then he said he felt bad and his stomach ached", Felipe's stepsister told the AP.
You give them food and water.
Nielsen reiterated Wednesday that six migrants, none of whom were children, died in the custody of Customs and Border Protection during fiscal year 2018.
Gomez Lucas said her father told her Felipe suddenly worsened. He was released from the hospital Monday afternoon, but taken back that night. A statement from CBP explained that he was taken to a hospital in New Mexico after an agent noticed signs of illness.
Apparently the most recent child to die did receive an initial medical screening.
Eight-year-old Felipe Gomez Alonzo was eager to go to the United States, the boy's stepsister said in a telephone interview Thursday, excited to attend school, find a new home, and buy clothes for his siblings.
In a secondary statement about the boy's death, CBP announced updated procedures for the children in its care.
Agustin Gomez Perez was 47 and in debt, and that path would only deepen his obligations.
Most families in Nenton, a mountainous area near the Mexican border, are of indigenous origin and subsist on corn and beans, as well as money sent back from relatives working in the United States and Mexico, according to a local government report.
House Democratic Whip Steny Hoyer of Maryland said in a statement Wednesday that the Democrats would hold hearings in the new Congress over Felipe and Jakelin's deaths, "as well as the conditions under which thousands of children are being held".
Oscar Padilla, the Guatemalan consul in Phoenix, confirmed that the father is still detained by the Border Patrol. We do not know what will happen.