An armed standoff in Springfield, Missouri, ended after 21 hours Monday night when police entered the suspect's home and found him and his two young children dead, authorities said.

Springfield police Lt. Eric Reece said Monday night that the suspect, identified as William R. Williams, 51, appeared to have shot his son, Brodie, 4, and his daughter, Marley, 2, and then himself. The Missouri State Highway Patrol, one of several law enforcement agencies that assisted in the confrontation, previously said the suspect was a civilian patrol employee in the early 1990s, but it would provide no other details.

Springfield police arrived at the apartment complex about 9:30 p.m. Sunday (10:30 p.m. ET) after they received a call asking them to check on the family. Williams was armed and refused to let them in, police said. Negotiations then proceeded for almost a full day as armed law enforcement officers ringed the building, even at one point sending in a robot to assess the scene.

Reece wouldn't describe how police entered the unit in detail, but he said several loud bangs heard by reporters Monday were related to their entrance. The building remained sealed Monday night, and no further information will be made public until Tuesday at the earliest, Reece said.

SOCIAL

STANDOFF: in every bad situation, find good! Adorable 4-year-old brings cookies & water to tnx @SGFPolice & @SGF_Fire pic.twitter.com/ZwinFnXaee — Chase Snider (@chasesnider) March 17, 2015

Barricade update on S. Cox: Special Reponse Team operations have been turned over to MSHP for now to relieve our officers. — Springfield PD (@SGFPolice) March 16, 2015

— M. Alex Johnson