DENVER — The driver who hit four Denver Police officers escorting a student walkout on Wednesday has not been arrested and no charges have been filed against him, authorities said Friday.

In a series of Tweets, police identified the driver as Christopher Booker, 41.

The investigation into the incident is “complex and will take time,” police said, adding that there were over 100 witnesses.

The four officers, all riding bicycles, were part of a large police force escorting hundreds of East High School students who staged a walkout and protest over the grand jury decisions in Ferguson, Mo. and New York.

Officer John Adsit was the most critically injured and has been in the hospital in critical condition. He has undergone multiple surgeries.

In a statement released by DPD, his family said that “surgeries originally scheduled for today are being postponed, to gain strength to continue his heroic fight for life.”

The family thanked the community for “overwhelming support and continued prayers.”

Police have said they believe Booker suffered a medical emergency that caused him to lose control of his black Mercedes and hit the officers. No other details have been released.

Thursday, the Denver Police Union claimed that several students cheered after the car hit them.

Police sources told FOX31 Denver that students were overheard cheering and chanting disrespectful terms after the officers were hit. The same sources say they saw teachers standing by while this happened, and they did not intervene.

Denver Public Schools has condemned any such action and said “All afternoon yesterday and all day today, students at East expressed their deep concern for Officer Adsit and his family and their appreciation for the police assistance in ensuring student safety during the march.”

Students in high schools in Aurora and Denver staged walkouts on Friday morning as well.