French authorities on Tuesday expressed worry that additional attacks could follow the shooting that killed four people at a Jewish school in the southern city of Toulouse Monday.The killer who is believed to have murdered seven people over the last nine days remains at large. Authorities said forensic tests proved the person used the same .45 caliber pistol in all three shootings and added they were worried another shooting might be carried out. "We are concerned about the possibility that they want to commit a fourth attack," French Interior Minister Claude Gueant told Le Figaro.Meanwhile, schools throughout France will observe a moment of silence on Tuesday at 11:00 a.m. local time in memory of the victims of the deadly shooting at the Ozar Hatorah school.Security at Jewish schools throughout the country remains tight while a large police presence was felt in in Toulouse, where two of the three killings ascribed to the spree killer took place. The third shooting took place in Montauban, a city 50 kilometers to the north.Eyewitnesses accounts have reported the suspect as being short and stocky. On two of the three shootings the person rode a black Model T-Max Yamaha motorcycle and wore a black helmet. One person said she saw he had a face tattoo. Le Point, a French weekly, reported on Monday that a group of former French soldiers discharged for having ties to neo-Nazi groups were suspects but the information could not be confirmed by French authorities. All the victims in the shootings were members of minority groups but police have so far not ascribed a motive to the the killer or released specific information on his or her identity.