The bloodshed showed once again the dangers of firearms in the United States, where 30 percent of adults say they own at least one gun.
The incident involving the two brothers occurred Sunday in Fort Worth, Texas where police told AFP the older boy found a gun in his home and fired the shot that killed his little brother.
"At this time, there will be no charges filed," police spokesman Jimmy Pollozani said in an email.
Several hours later, a six-year-old boy was seriously wounded in nearby Arlington, Texas, by a bullet probably fired by one of his brothers.
Several children were playing in a bedroom when a boy aged about 10 found a rifle, pointed it and pulled the trigger, Christopher Cook of the Arlington police told the Dallas Morning News.