A 10-year-old girl from El Salvador has died in US custody, it has emerged, bringing the total number of migrant children to have died after being detained by border authorities in the last eight months to six.

Mark Weber, a spokesperson for the US Department of Health and Human Services (HHS), said the girl was “medically fragile”, with a history of congenital heart defects. She died in September 2018.

He said the child entered the custody of an Office of Refugee Resettlement in San Antonio, Texas, on 4 March 2018. He said that complications from an unspecified surgical procedure left her in a comatose state.

Officials said she was released from the hospital in May and sent to a nursing facility in Phoenix, Arizona, for palliative care.

She died on 29 September of fever and respiratory distress in a hospital in Omaha, Nebraska. Officials said she had been moved to Omaha to be closer to family.

The girl’s name, as well as when and how she entered the US, has not been disclosed. HHS provides care to children the government considers unaccompanied.

Democrats are calling for investigations into her death, and the five other reported deaths of migrant children detained by US border patrol.

Representative Joaquin Castro, the chair of the Congressional Hispanic Caucus, accused the Trump administration of covering up the 10-year-old girl’s death.

“It’s outrageous that another child has died in government custody and that the Trump administration didn’t tell anybody,” the Texas Democrat told CBS. “They covered up her death for eight months, even though we were actively asking the question about whether any child had died or been seriously injured.”

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“We give them billions of dollars, and they want to use it on a wall instead of spending it to make sure that people don’t die and that they can medically treat emergencies that migrants maybe come into or that their own agents may come into,” he continued.