(Spotsylvania)- State police are investigating what caused four crashes involving 20 vehicles on Interstate-95 north in Spotsylvania closing all of the northbound lanes. The accident happened two miles north of Thornburg just before noon yesterday. Nine people with non-life threatening injuries were taken to three different medical facilities for treatment. All of the lanes of the interstate reopened just before 3 p.m., but traffic was backed up almost ten miles at that time and took awhile to clear. 