Who are الراسخون فی العلم in verse 7 of Surat Al 'Imran?

In the seventh ayah of Surat Al 'Imran, reference is made to "الراسخون فی علم" (those who are firmly rooted in knowledge): >هُوَ الَّذِيَ أَنزَلَ عَلَيْكَ الْكِتَابَ مِنْهُ آيَاتٌ مُّحْكَمَاتٌ هُنَّ أُمُّ الْكِتَابِ وَأُخَرُ مُتَشَابِهَاتٌ فَأَمَّا الَّذِينَ في قُلُوبِهِمْ زَيْغٌ فَيَتَّبِعُونَ مَا تَشَابَهَ مِنْهُ ابْتِغَاء الْفِتْنَةِ وَابْتِغَاء تَأْوِيلِهِ وَمَا يَعْلَمُ تَأْوِيلَهُ إِلاَّ اللّهُ وَ**الرَّاسِخُونَ فِي الْعِلْمِ** يَقُولُونَ آمَنَّا بِهِ كُلٌّ مِّنْ عِندِ رَبِّنَا وَمَا يَذَّكَّرُ إِلاَّ أُوْلُواْ الألْبَابِ >He it is who has revealed the book to you; some of its verses are decisive, they are the basis of the book, and others are allegorical; then as for those in whose hearts there is perversity they follow the part of it which is allegorical, seeking to mislead and seeking to give it (their own) interpretation. But none knows its interpretation except allah, and **those who are firmly rooted in knowledge** say: We believe in it, it is all from our lord; and none do mind except those having understanding. ([Surat 'Ali 'Imran 7](http://quran.com/3/7)) Who is this phrase referring to?