Technicians with the South Coast Air Quality Management District took air samples in Riverside and San Bernardino counties to make the determination.
"We now have solid evidence that clearly points to the Salton Sea as the source of a very large and unusual odor event," said Barry Wallerstein, AQMD executive officer, in a statement.
Officials found that there was a clear progression of hydrogen sulfide levels, with the highest concentrations found at the Salton Sea.
Hydrogen sulfide, which is found in the Salton Sea, has a rotten-egg odor. Scientists believe strong winds pushed surface waters aside and allowed water from the bottom of the sea - rich with decaying and odorous bacteria - to rise to the surface.
Officials said on Sunday night, a strong thunderstorm developed over the Salton Sea, and winds pushed odors from the Salton Sea to the northwest, across the Coachella Valley, through the Banning Pass and across the L.A. Basin.
Officials say hydrogen sulfide concentrations at the Salton Sea were higher than normal on Monday, but not high enough to cause harm to human health.
AQMD said it received about 235 calls about the smell since midnight Sunday.