Mexico's missing students: international investigators say they are being obstructed

[DetlefK]

1. I have read a theory that the students unwittingly stole a bus smuggling heroin.

http://www.spiegel.de/panorama/justiz/mexiko-die-wahrheit-ueber-die-vermissten-studenten-soll-nicht-ans-licht-a-1078586.html
(article in german)

The federal attornies are preventing the GIEI from questioning the soldiers that were stationed nearby.

And the bus the students used? There were 5 buses. The students had stolen that fifth bus. The existence of that bus was deleted from official reports.
Why?
What could possibly be interesting about a long-distance bus travelling from Iguala to Chicago?
Why could someone possibly have an interest in not talking about that particular bus?

The article above tells of a theory that the students stole a bus that was used to smuggle heroin. And when somebody stole their drugs, the narcos and corrupt cops panicked. Killing the students was just the cover-up for an operation to get their heroin back.

The GIEI asked Chicago for aid and additional information about this kind of drug-smuggling.
It took the mexican federal attornies two months to pass the letter on to the US.