The oil has now been contained, but removing all the oil from the water could take months.
Officials at the scene agreed that floodwater from the swollen Little Rock River played a part in causing the cars to leave the tracks, but said they weren't yet sure whether the waters compromised the track, physically pushed the cars off it or played a part in some other way.
Williams says he doesn't yet know how much oil has leaked and how many cars are leaking.
Almost half - 100,000 gallons - of the spilled oil has been contained with booms near the site, but some of the crude oil did make its way into the floodwaters and nearby rivers.
"In addition to focusing on the environmental recovery, ongoing monitoring is occurring for any potential conditions that could impact workers and the community and so far have found no levels of concern", Williams said.
Crews are trying to determine how quickly crude oil leaking from derailed railroad tanker cars in northwest Iowa will reach cities downstream.
32 oil tanker cars derailed on Friday 22 June in Lyon County, Iowa.More news: Korea to express regret over refereeing in match
Cleanup of an oil spill caused by the derailment of almost three dozen oil tankers in northwestern Iowa has begun.
Some officials have speculated that floodwaters eroded soil beneath the train track. The river rose rapidly after 13 to 18 cm of rain fell in the days prior to the derailment.
(Rock Valley) - Iowa Governor Kim Reynolds (R) visited the flooded ares of northwest Iowa Saturday afternoon. ConocoPhillips spokesman Daren Beaudo said each tanker can hold more than 25 000 gal. of oil.
BNSF railroad spokesman Andy Williams said no one was injured when 33 oil tanker cars from Alberta, Canada, derailed around 4:30 a.m. Friday just south of Doon in Lyon County.
The spill threatened to contaminate drinking water for residents about 150 miles (240 km) downstream in Omaha, Nebraska.
City public information officer Travis Olson says the wells were shut down as soon as Rock Valley officials were told of the derailment and oil spill north of them early Friday morning. It plans to drain and clean its wells and use a rural water system until testing shows its water is safe.