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"The beaches are closed until we make a determination that it's safe,'' U.S. Coast Guard Capt. Charlene Downey said at a late-morning news conference.
Earlier, the Los Angeles County Department of Public Health had indicated that the beaches could reopen some time today if no more tar turned up in the water or on the sand.
The closure affects nearly all South Bay beaches from El Segundo to Redondo Beach.
The environmental group Heal the Bay earlier had warned that the beaches should not be reopened "until all the oil is cleaned up.''
"Heal the Bay has concerns about opening the beaches and even allowing people on the sand between the lifeguard towers and the water,'' said Sarah Sikich, one of two scientists the group deployed to the scene.
"It's nearly impossible to walk along the beach in that area without encountering a small oil glob, and from a human health perspective, exposure through skin contact is a concern.''
Sikich also warned that children could put contaminated sand in their mouths and that blobs had been found outside the closure area in Playa del Rey.
Crews began conducting flyovers in the area Thursday morning to try to determine the source of the substance. At an afternoon briefing on Thursday, Downey said "significant progress'' had been made overnight in the cleanup effort.
"Our teams have worked ... on the ground and in the air and on the water, (and) have reported that there appears to be no new tar balls or anything additional to the amount that we have recovered thus far,'' Downey said.
"Our air crew did observe a sheen off the South Bay, which from what we understand is typical for that area,'' Downey said. ``But what we believe to be pretty good news is that it doesn't appear to be any in the surf, the surf line, (like) we saw (Wednesday).''
A loon covered with oil was brought to the incident command post in Manhattan Beach on Thursday and was in stable condition under the care of the Oiled Wildlife Care Network, the U.S. Coast Guard reported. It was unclear where the bird was found and how it became oiled. There has been no other indication, however, that wildlife has been harmed.
Authorities released a telephone number -- (877) 623-6926 -- for people to call to report the location of any wildlife found to be in distress from the effects of the substance.
Officials warned the public not to go into the ocean, or to use the beaches "from the lifeguard towers to the waterline.'' The beaches from the lifeguard towers to the inland side are OK for the public to use, however, officials said.
The unknown substance began hitting the beach about 10 a.m. Wednesday.
The U.S. Coast Guard and other agencies collected samples of the material and were trying to identify the substance and its source. Results were pending.
A company known as Ocean Blue Environmental was hired to clean up the mess.
Recorded information on beach conditions is available 24 hours a day on the county's beach advisory hotline by calling (800) 525-5662 or online at www.publichealth.lacounty.gov/beach.
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