。
Rumors of the riot had been spreading.
Literal
Riot [possessive-の] rumor [subject-が] was-spreading.
広まっていた is the past progressive of 広まる (to spread, intransitive). ~ていた marks an ongoing state in the past — the spreading was already underway. Note the intransitive 広まる rather than transitive 広める — the rumor spread on its own, with no stated agent.