lots of people think it was nixon, i tend to think now that Nixon was pushed around and set up in a lot of ways. the Cia folks around him and his political backers seemed to have put him in no win positions. He may have even fear for his life.
But here's a link to Hunts sons book.
Bond of Secrecy E. Howard Hunt book
you can down load it as a PDF.
he mentions several things that make Nixon look guilty as sin.
30+ FBI agents there within minutes , FBI taking Air controllers tapes, Several high level reappointments to the FAA and another justice position to "oversee" the investigation are the main one if memory serves.