What pieces of evidence do investigators have access to in the MH17 crash investigation?
Air Accident Investigation

Evidence available to the air accident investigators can include


Flight data recorder.
Cockpit voice recorder.
Aircraft wreckage.

residues, burn marks, punctures, tears and evidence of stresses.

ATC communication recordings and controller statements.
Airline records. 

Aircraft maintenance records
Pilot schedules, dates of health checks, training, hours on type etc.

Radar and ADS-B data (position, speed, altitude etc).
Technical data transmitted in flight to airline, engine maker etc.
Photographs, measurements and other reports from investigators at crash site.
Eye-witness accounts from people on the ground.
Relevant NOTAMs.
Data from other aircraft in the vicinity at the time and in previous and following days (e.g. typical tracks and variations over eastern Ukraine).


In the case of MH17 the situation is obviously made more difficult by the armed conflict at the crash site and the wider political conflicts. Investigators have to take into account that some evidence may have been tampered with, destroyed or fabricated.

As others have noted, the purpose of an air-accident investigation is not to determine legal liability. Where needed, there is an entirely separate criminal investigation for that purpose.

Until they publish their report, we won't know whether the evidence available to them is sufficient for investigators to make any recommendations for changes by manufacturers, operators or regulators to reduce the number of future future occurrences of this type of incident (e.g. recommending airlines don't fly at any height over areas where there is armed conflict).

Other Investigations

Evidence relevant to other types of investigation might include


US records of alleged missile launches detected by strategic/tactical missile detection satellites.
Ukrainian alleged intercepts of separatist radio communications.
Online material allegedly published by Ukrainian separatists.
Online photos and videos alleged to be of Russian missile launchers in transit through the Ukraine-Russia border regions.


It is difficult to see how the conditions could ever be conducive to a satisfactory judicial investigation of this material acceptable to all parties with an interest in the incident.