A freight train carrying hazardous materials derailed in southwestern Wisconsin on Thursday, injuring four employees and sending two containers into the Mississippi River.

Lena Kent, a spokesperson for the train's operator, BNSF Railway, told NPR that "the volumes involved don't pose a risk to the river or nearby communities." A boom, which helps absorb and contain spills on water, has been placed in the area as a precaution.

The incident occurred around 12:15 p.m. local time in DeSoto, a village near the border of Wisconsin and Iowa. Two of the three train engines, as well as 10 cars, went off track to some degree, she said.

All four crew members were transported to a hospital for minor injuries and have been released as of Friday morning, according to Kent.

The cause of the derailment remains unclear.

The train, destined for Chicago, was carrying lithium-ion batteries, paint and oxygen containers — all of which are flammable and can pose a threat to the environment.

According to Kent, the two containers that went into the Mississippi River did not contain any hazardous materials — though some cars that contained lithium-ion batteries landed on the shore.

BNSF is working with local and state agencies to investigate the cause, as well as make arrangements to remove the derailed cars from the site.

The National Transportation Safety Board is aware of the derailment, but is not involved in the investigation, Jennifer Gabris, an NTSB spokesperson, told NPR.

On Thursday evening, Crawford County Emergency Management, who responded to the incident, wrote on Facebook, "There is currently no harm to the community or environment, but Desoto Fire Department is working closely with BNSF staff throughout this clean-up process to prepare for any unknown."

Meanwhile, the main track remains blocked in both directions and it is unclear when it will reopen.

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