Cleanup of "slippery substance" on I-210 causes lengthy traffic backup in Pasadena

An hours-long cleanup of an unknown substance on the 210 Freeway caused a lengthy backup of traffic in Pasadena on Monday evening. 

The work began at around 3 p.m. on westbound lanes of I-210 near Lake Avenue, according to the California Highway Patrol, who issued a SigAlert due to the cleanup. 

Though they only expected the road to be closed for around two hours, the closure extended well past 7:30 p.m., with the two rightmost lanes still closed. 

Officers have not yet disclosed what the material is, but told KCAL News that it was a "slippery substance" that caused the closure because it caused an "immediate hazard."

As of 8 p.m., they were still unsure how long the Caltrans cleanup crew would remain at the scene. 

This is a developing story. Check back for details.