。
The riot became uncontrollable.
Literal
Riot [topic-は] hand [subject-が] can't-be-attached became.
The idiom 手がつけられない ('uncontrollable, hands can't be put to it') appended with なる expresses transition: the riot moved into an uncontrollable state. ~なくなる marks the onset of a new negative state — things went from manageable to unmanageable. The verbal なった captures the change of state, distinct from the static description 状態だった ('was in such a state').