Police say at least six people are dead after a school bus and a commuter bus crashed in southwest Baltimore. The Baltimore City Schools bus driver was killed in the smash when his vehicle plowed into the back of the Maryland Transit Administration bus at around 6.30am on Tuesday. There were also 'significant injuries' in the horrific crash which saw at least five hospitalized - one of whom is in critical condition. Several passengers were suffering from shock. Police confirmed that no children had been aboard the school bus at the time. Emergency crews are still on the scene of the deadly crash which occurred just outside the National Cemetery Loudon Park, in Southwest Baltimore, on Frederick Avenue. The road is closed and will remain so for 'a significant period of time' officials said. Baltimore Police spokesman T.J. Smith said police are investigating the cause of the 'absolutely tragic situation that occurred this morning.' But he said a medical examiner will be able to determine a working theory that the school bus driver may have suffered a 'medical emergency.' 'We also have a surviving vicitm that was on the yellow bus,' he added at the morning press conference. 'We will be able to interview her to find out more.' The school bus had been driving to pick up its first student of the day, who lives just four or five blocks away from the crash site, when it crashed into the MTA vehicle. The yellow bus then kept going, striking a pillar before finally coming to a stop several feet away. Smith said there were no skid marks on the road - indicating that the driver never slammed on his breaks to avoid the collision. 'Something catastrophic took place,' he added.