A 3-year-old migrant girl who was on a Texas-sponsored bus to Chicago died last week, prompting investigations by the US Centers for Disease Control and Prevention and other agencies, authorities said.

The child, an asylum seeker who died Thursday, presented "health concerns" while en route to Chicago from Brownsville in south Texas, the Texas Division of Emergency Management said in a news release Friday.

The bus pulled over and security personnel on board called 911 before the child was then taken to a hospital, where she died, the news release said. The Texas office did not specify what symptoms the child was exhibiting when the bus pulled over.

"CDC is working with other federal agencies and state and local public health authorities to investigate the tragic and heartbreaking death of a small child onboard a bus in Illinois, and we are conducting follow-up with other passengers," CDC spokesperson Nick Spinelli told CNN in an email.

A preliminary autopsy on the girl was inconclusive, Marion County, Illinois, Coroner Tory Cannon said, adding that "microscopic studies of tissue samples may provide a definitive cause of death."

Before boarding the bus, no passenger presented with fever or other medical concerns, the Texas Division of Emergency Management said.

"After being processed by U.S. Customs and Border Protection and the City of Brownsville, each bus passenger underwent a temperature check and was asked if they had medical conditions that may require medical assistance," the agency's release said.

The migrants had willingly chosen Chicago as a destination and signed consent waivers before boarding, the Texas office added.

Under Texas Gov. Greg Abbott's border initiative -- known as Operation Lone Star -- the state has bused more than 30,000 migrants to cities across the country since last spring, according to a Friday news release from Abbott's office. The effort is a protest of President Joe Biden's immigration and border security efforts, which Abbott has called inadequate.

More than 4,900 migrants have been bused to Chicago from Texas cities since August 2022, the release said.

CNN reached out to Abbott's office for comment and was directed back to the state's emergency management division.

"A loss of any child is devastating and heartbreaking," White House Press Secretary Karine Jean-Pierre said. "Our condolences goes out to the child's family. That is horrific news."