The man killed one woman, apparently his cousin, then went to another home and shot his longtime girlfriend, their four children and himself, Capt. Rich Lockhart said.
Police did not immediately identify the shooter or the women. Relatives identified the shooter's girlfriend as Shanika King, who was in her 30s.
The slain children were a girl, 14, and two boys, ages 1 and 11, police said. An 8-year-old boy was in critical condition.
Witnesses identified the first victim, a woman in her mid-30s, as the shooter's cousin, Lockhart said. The woman's son told police a man came to the door and asked to see his mother. The boy said he heard a "pop," but thought it was fireworks.
After the man left, the boy found his mother had been shot, Lockhart said. About 15 minutes later, police received a call from a woman who said her son-in-law had called her to say he had shot her daughter and grandchildren.
Two of the couple's other children had spent the night at their grandmother's house, said King's aunt, Janna Walker, who lives nearby in the neighborhood, about five miles east of downtown Kansas City.
Kim Drew, a neighbor whose 13-year-old son was a friend of the slain children, described the shooter as a controlling man who wanted things done his way.
"He didn't want her to smoke," Drew said. "He ran things and he was bossy. That much I did see."
Lisa Robinson, who worked with Shanika King at a hotel for about five years, said King always put her children first.
"She was a good lady," Robinson said. "She would go without anything to make sure her kids had what they need."
The uncle of the first woman killed called the shootings "an extraordinary tragedy."
"This is like being in the twilight zone," Wilbert Blackmon told The Kansas City Star.
He said his niece was a "fine young lady" who had four children, and that there had not been any indication of conflict or hint of impending violence in the family.