JUDY WOODRUFF:

A major spill of toxic coal ash is raising questions again about the safety of water and the government regulators overseeing industry. This time, the accident was reported in North Carolina on February 2, specifically in the Dan River in the city of Eden.

The Duke Energy company said more than 50,000 tons of coal ash had leaked from a containment pond into the river, as seen in these photos taken by the Waterkeeper Alliance. Parts of the river look like a gray sludge. The ash, which contains arsenic and other dangerous metals, is produced from burning coal.

Today, at a state hearing, Duke Energy officials apologized for the spill and said they would clean it up. But big questions remain, including about coal ash containment ponds elsewhere.

Reporter Michael Biesecker has been covering this for the Associated Press. And he joins us now from Raleigh.

Michael Biesecker, first of all, tell us what caused the spill. What is known about that?