There are indeed limits not to cross in the investigation. An interview in itself doesn't cross those, but there are ways it could go too far. Sleep deprivation, repeatedly asking the same questions, the usual sort of harassment-style interrogation used by police is almost certainly off-limit. OTOH, just a polite meeting with questions asked, not very different from what you'd get at a job interview or any other type of background check, should be just fine.
I didn't see any evidence in the article (biased as it is) that lines were crossed here