By Saturday evening, the oil flowing out of Chevron's busted Crude Oil Pipeline No. 2 was down to around five gallons a minute, a trickle compared with the 50 gallons a minute that were reported to be spilling earlier in the day.

What remained a mystery: When did the leak begin? And why, with monitoring equipment in place on the pipeline, did it apparently take hours to learn of the break?

"We will get to the bottom of how this happened," Salt Lake City Mayor Ralph Becker said. "And we will address necessary measures to make sure the community continues to be protected in the future."

An early report from city officials indicated the initial leak took place about 10 p.m. Friday. But that report later was withdrawn and blamed on miscommunication between Chevron and the city.

"We do not know yet when the leak first happened," said Dan Johnson, a spokesman for Chevron Corp. "Our first and most important priority was to get the leaked stopped and the damage contained."

Without knowing when the leak started, though, any estimate of the actual size of the spill remains just that, said Becker's spokeswoman, Lisa Harrison Smith. Still, Fire Department Deputy Chief Karl Lieb estimated 500 barrels (about 21,000 gallons) of oil escaped.

The residual leakage represented the crude that remained in the 10-inch pipeline after it was shut down. The valve used to stop the flow -- soon after Chevron learned of the leak at 7:42 a.m. Saturday -- was about seven miles east of the break.

"Our pipeline-monitoring system was active but did not identify the source of the leak," company spokesman Mark Sullivan said. "Our investigation will examine that and report on the findings."

He said the company would assume full responsibility for any "financial damage, environmental damage, safety concerns, impacts on health ... and cleanup."

Becker vowed to hold the company to that pledge.

Johnson said a team was being flown into Utah to assess the damage. Also expected to arrive are representatives from the oil company's insurance carriers, who will begin contacting those affected by the leak.

State records indicate that an earlier leak on the 52-year-old pipeline occurred in February 2002. During that leak, blamed on corrosion, an estimated 207 barrels spilled. Damage was estimated at nearly $318,000.

A leak also took place near Park City in August 2004. During that incident, the result of excavation damage, around 470 barrels leaked. That damage was pegged at $442,000.

Sullivan said the pipeline must be inspected every five years. It was last checked in 2008. The U.S. Department of Transportation and the Environmental Protection Agency monitor the pipeline.

"The leak was on the downhill side," Sullivan said, "where some of the standard monitoring tools don't work as well as other monitoring tools."

Sullivan said he couldn't speculate on what caused the leak, but water corrosion usually is the culprit when pipes break.

The oil being transported on Chevron's pipeline was a medium-grade crude, which refers to how easily the oil flows. Light crude flows almost like water while heavy crudes are closer to the consistency of furniture wax, or petroleum jelly, and must be heated before they flow easily.