Illinois’ attorney general has asked the state’s highest court to review what prosecutors consider a too lenient sentence for the white Chicago police officer who shot dead black teenager Laquan McDonald.

Attorney general Kwame Raoul and the special prosecutor who won the conviction against Jason Van Dyke said they believe Judge Vincent Gaughan did not properly apply the law when he sentenced Van Dyke to six years and nine months in prison.

In a rare move, Mr Raoul and Kane County state’s attorney Joseph McMahon filed a request with the Illinois Supreme Court seeking an order that would send the case back to Mr Gaughan for a new sentence.

Without a new sentence and with credit for good behaviour, Van Dyke is likely to serve only around three years for firing 16 bullets into McDonald in 2014.

Dashcam video of the shooting released by the city in 2015 showed Van Dyke continued to fire as the 17-year-old crumpled to the street and lie on the ground. The sentence was the first imposed on a Chicago police officer for an on-duty shooting in half a century.

Monday’s move is the latest chapter in an ongoing saga that has included massive demonstrations, the firing of the police superintendent by the mayor and the ouster of the county’s top prosecutors by voters a few months later, the judge’s ruling to sentence Van Dyke to less than seven years in prison was met with anger and disappointment by activists.

Mr Raoul reacted quickly. In one of his first moves after taking office in January, he announced he would review the sentence Mr Gaughan had handed down just days earlier.