CONTRA COSTA COUNTY (CBS SF) — Marine patrol units continued their search Friday for Umut Felik, a San Francisco man who disappeared while swimming in the Delta, authorities said.

In an email, Contra Costa County Sheriff’s spokesman Jimmy Lee said marine units from several agencies were again searching the murky waters of the Delta for any sign of Felik.

READ MORE: Growth of Willow Fire Near Big Sur Slows Overnight, Still Zero Percent Contained

Deputies received a call Thursday morning that a boater who had gone into the waters in the San Joaquin River north of Webb Tract and could not be located.

Felik, a 27-year-old from San Francisco, was swimming when he apparently had trouble staying above the water. At one point he went underwater and did not resurface.

READ MORE: UPDATE: 1 Dead, 6 Wounded in Saturday Evening Shooting at Oakland's Lake Merritt

His friends on a boat went into the water to search for him, but were not able to find him and called the Sheriff’s Office.

The Contra Costa Sheriff’s Office Marine Patrol Unit responded and were assisted by Solano County, Sacramento County, San Joaquin County and the U.S. Coast Guard.

A search of the area with a sonar was suspended at 8 p.m. Thursday with Felik still missing.

MORE NEWS: San Francisco Police Arrest Suspect In String Of Brazen Mission District Pharmacy Retail Thefts

Anyone with any information on this incident to asked to contact the Contra Costa Sheriff’s Office Marine Patrol Unit at (925) 427-8507.