SALTON SEA, Calif. -

A strong odor first noticed throughout Riverside County on Sunday night hung in the air today, but authorities were at a loss to explain it.

The Coachella Valley has been experiencing the smell for the past few weeks, and the explanation given, was due to massive fish die offs and the algae blooms that have caused them at the Salton Sea.

On Monday the South Coast Air Quality Management District, which monitors pollution levels in the region, released a statement saying the agency's inspectors were attempting to track the origin of the odor, following more than 100 calls about it.

"Residents have complained from a very wide area including the Inland Empire and much of the Los Angeles Basin," according to an AQMD statement. "Fish kills, algae blooms and other biologic conditions in lakes can cause strong odors. Industrial facilities such as waste water plants also can cause sulfur odors. At this time, AQMD hasn't confirmed any source as the cause of the widespread odor."

The rotten-egg odor wafted into Riverside around 10 p.m. Sunday and was reported in Murrieta, Calimesa and in parts farther west.