A recall of romaine lettuce that has sickened students with E. coli poisoning is expanding as the government tries to find out where the contamination occurred.

The Food and Drug Administration said late Monday that a food distributor in Moore, Okla., is recalling romaine lettuce that came from the same farm in Yuma, Ariz., that grew lettuce that sickened students in Michigan, Ohio and New York. Ohio-based Freshway Foods announced a 23-state recall of romaine lettuce last week related to those outbreaks.

At least 19 people have been sickened in connection with the E. coli outbreaks, which come from a rare strain of the disease that is difficult to diagnose. The federal Centers for Disease Control has said they are looking at another 10 probable cases of E. coli poisoning in connection with the tainted lettuce.

The FDA said it is investigating the Yuma farm from which the romaine lettuce was harvested and is attempting to determine the point in the supply chain where the contamination occurred. The agency declined to name the farm that grew it.

Many of those sickened were students at colleges and universities in the three states. Middle and high school students in New York were also sickened, including a 15-year-old and a 17-year-old who developed hemolytic uremic syndrome, which can cause bleeding in the brain or kidneys. Local health authorities in Dutchess County, where the students fell ill, said they are all expected to make a full recovery.

Most of the lettuce recalled was sold to food service establishments. It does not affect bagged lettuce in the grocery store.

A spokeswoman for California-based Andrew Smith Co. said on Monday that the company is recalling lettuce sold to Vaughn Foods in Moore, Okla. and also to another distributor in Massachusetts. Spokeswoman Amy Philpott would not give the name of that distributor because the lettuce is already past its expiration date, she said.