A 22-year-old Kansas City man is charged with killing his ex-girlfriend, her toddler son and her new boyfriend at a south Kansas City home.

Joseph L. Nelson was charged Thursday with three counts of first-degree murder, three counts of armed criminal action and one count of first-degree burglary.

The victims of the Sept. 8 shooting were 17-year-old Bianca R. Fletcher; her 1-year-old son, Joseph; and 18-year-old Shannon Rollins Jr., Fletcher's boyfriend.

Nelson was arrested at his home on Wednesday.

Court documents indicate Nelson told witnesses that he "lost it" when Fletcher threw a diaper box at him during an argument. Documents say he told police he killed Rollins and the baby because they were witnesses.

It was not immediately clear if Nelson has an attorney.