So … that doesn’t mean there has to be joint investigation . This is international norm … wherever the accident takes place, it is the local authority that investigates the accident. Though the country whose citizens have been affected can certainly ask for information on the case !!!
Well Pakistan is nobody to question why a suspect has been released or arrested with a case that happened in India … though they surely can ask for information. This is the way things are done in civilized countries … perhaps it is time you learn about it.