Yoshihide Suga, chief cabinet secretary, told a news conference: "This is a very heart-wrenching and shocking incident in which many innocent people became victims."
The Japan Times reported that some residents and staff may have been tied up during the attack. The newspaper said that hospitals reported many of the injured had wounds to their necks.
Japanese radio station NHK said the suspect broke into the building by smashing a window.
Officials said Uematsu worked at the centre between December 2012 and February this year and left for "personal reasons".
According to the centre's website, it holds a maximum of 160 people including staff.