(Astoria, OR) -- State and federal officials say crews have completed clean-up of tar balls on the Oregon coast. The petroleum-based substance began washing ashore in mid-May, affecting birds and other wildlife. Late last week, crews took advantage of low tides to survey beaches after stormy weather and no new oil was found. They have still not determined the source of the contamination. Over the last several weeks, crews collected almost a ton of oily debris across 36 miles of beach in Oregon and Washington.