Following a noun, [...] dei ricercatori translates as "[...] of the researchers" / "the researchers' [...]" (note the apostrophe at the end). It implies possession.
Technically, I think your translation "They are from the researchers", could work as well, when replying to someone asking to whom [some objects] belong. See marziotta's answer to Elena18's (similar) question.
However, the construction sono dei/degli/delle [...] commonly translates as "they are (some) [...]", it is used a lot in Italian and should be understood as a concept that does not always translate word for word. Examples: http://context.reverso.net/traduccion/italiano-ingles/sono+dei
When talking about files or other things that "come from the researchers", vengono dai ricercatori might be more natural to use: http://context.reverso.net/traduccion/italiano-ingles/vengono+dai
In case you're struggling (like I was) to understand the difference between di(+definite article) and da(+definite article), the following links provide examples of the use of dei http://context.reverso.net/traduccion/italiano-ingles/dei+bambini and dai http://context.reverso.net/traduccion/italiano-ingles/dai+bambini in a context.
Also not native, but for the French speakers, "dei" seems to be similar to the French "des" which can be completely left out in the translation as here, or can mean "some" or even "of the" but not just "the", but unlike English, is required. Not sure how far this comparison can be drawn made but thought it might help.
No, that is not what Elena18 means, I think. She writes an apostrophe after the researchers, so it is the researchers' (genitive of the researchers). There are also quotes around it, "researchers'", so it's easy to miss. The example she gives actually makes it clear what she means, but all answers so far seem to be ignoring this. How would you say "Whose books are those? They are the researchers'." in Italian? I think I would also guess the translation that Elena is giving, perhaps wrongly.
"They are the researchers" suggests that they are all of the researchers. I think that 'dei' leaves room for there to be other researchers, since it suggests "some of", so "They are researchers" (which also leaves room for there to be other researchers) is more accurate.
I'm not sure whether "They are some of the researchers" would have been marked correct, but I bet it would.
l'avvocato (male) / l'avvocatessa (female)
il ricercatore (male) / la ricercatrice (female)
l'architetto (male) / l'architetta (female)
il direttore (male) / la direttrice (female)
il meccanico (male) / la meccanica (female)
il segretario (male) / la segretaria (female)
l'operaio (male) / l'operaia (female)
il capitano (male) / la capitana (female)
il pescatore (male) / la pescatrice (female)
il contadino (male) / la contadina (female)
il poliziotto (male) / la poliziotta (female)
il dottore (male) / la dottoressa (female)
lo scrittore male / la scrittrice (female)