According to a ruling released on Thursday, Hennepin County Judge Peter Cahill said prosecutors did not have probable cause to charge Chauvin with third-degree murder but said the evidence supported taking to trial a second-degree murder charge, which could lead to decades in prison.

Minnesota Governor Tim Walz announced he had activated the Minnesota National Guard as a "precautionary step" following the ruling, which he said marked "a positive step in the path toward justice for George Floyd".

The fatal arrest of Floyd on May 25, captured on video by bystanders, sparked weeks of angry and sometimes violent protests in Minneapolis and elsewhere over police brutality and racism. Chauvin is white, while Floyd was a black man.

Cahill described in detail how Chauvin used his knee to press Floyd's neck and face "into the unforgiving concrete of Chicago Avenue" in Minneapolis even after Floyd pleaded that he could not breathe and then became unresponsive.

Cahill upheld a manslaughter charge against Chauvin and all six charges against three other officers - Thomas Lane, J Alexander Kueng and Tou Thao - for allegedly aiding and abetting Chauvin in the murder. Kueng and Lane helped hold Floyd down by restraining his back and legs, Cahill noted, while Thao kept a group of bystanders at bay.

Earlier this month, Cahill ruled that Chauvin, who had posted a $US1 million ($A1.4 million) bond with a condition he stay in Minnesota, could leave the state as he awaited trial due to safety concerns.

Australian Associated Press