We now know some of what happened inside a Chester apartment where four people died. Some, but not nearly all.

And that’s what is proving so frustrating about this case. I can only imagine what the family members of those who died inside that apartment on Keystone Road must be dealing with.

The truth is we may never just what happened.

After a press conference held yesterday by Delaware County District Attorney G. Michael Green and Medical Examiner Dr. Fredric Hellman, here’s what we know.

Lindsay Cassidy, 23, and her 3-year-old son Kolby O’Brien, were strangled. Cause of death is listed as asphyxia by strangulation. Hellman ruled them both homicides.

All four people are believed to have died March 28. Concerned relatives who had not heard from loved ones alerted police, who found the four bodies in four different rooms inside.

The deaths of the other two people found in the apartment, Michelle Fynes, 22, and John Michael Bryant, 28, are listed as drug overdoses. Hellman indicated both had fatal levels of cocaine and PCP in their blood. Drugs also were found in the system of Cassidy.

Hellman made no ruling in the two drug deaths, as to whether they were accidental overdoses or a possible suicide.

That joins a list of the things we still don’t know about this case, other than the fact that four lives were lost inside that apartment.

We don’t know who killed Cassidy, or who snuffed out the angelic little face of 3-year-old O’Brien. A ligature mark was found on the toddler’s neck. And we don’t know the precise circumstances that led to the fatal ODs involving Fynes and Bryant.

One thing was stressed by Green. He does not believe there is a killer on the loose. He believes whatever happened inside that apartment involved the four people inside.

The investigation is continuing. That’s good. Because there’s still a lot we don’t know about this baffling case.