NEW : At least six people were killed in the accident , France 's president says . Comment: France 's national railways company says the derailment was an accident . The cause of the derailment is being investigated , minister says .
NEW : At least six people were killed in the accident , France 's president says . Last four cars of train derailed , the railway company says . France 's national railways company says the derailment was an accident . The cause of the derailment is being investigated , minister says .