Two young girls have died and a boy was hospitalized in critical condition in the Pacific Northwest on Wednesday after becoming infected with E.coli in two separate incidents, health officials said.

Investigations have been launched into the source and strain of the bacteria that killed a 3-year-old girl in Lynden, Washington, and a 4-year-old girl, Serena Profitt, in Otis, Oregon.

A third child, Bradley Sutton, 5, has also tested positive for E.coli and was being treated at Mary Bridge Children's Hospital in Tacoma, Washington, a hospital spokeswoman said.

Bradley became ill after his family traveled to Oregon over the Labor Day weekend, where he swam in a pond and shared the same food as 4-year-old Profitt, his relatives told investigators.

Profitt died on Monday from a form of kidney failure called hemolytic-uremic syndrome, or HUS, which can develop in young children or the elderly after exposure to certain extremely dangerous forms of E.coli, according to the Mayo Clinic.

Medical officials believe Serena and Bradley's infections likely stemmed from the same source since the children were playing together and shared food at a restaurant.

The 3-year-old in Washington, who was not named, had no connection to the Oregon exposures, but it was still unknown if a common food item was involved, officials said.

"The thing we are trying to focus on is if there is a single source that we can pinpoint and the public is at risk," said Dr. Greg Stern of the Whatcom County Health Department.

Cultures from the Lynden patient were being analyzed by state health officials and the Centers for Disease Control and Prevention, he said.

Medical authorities in Oregon have not yet confirmed what strain or source was involved, said Tim Prudhel, a spokesman for Lincoln County Health and Human Services.

"It's a high-priority, active investigation," he said.

Possible sources of the E. coli infection include high-risk foods such as undercooked meat, unpasteurized milk or juices, restaurants at which cases have eaten, exposure to live animals and recreational water.

Most of the multiple strains of E. coli are harmless or cause relatively brief illness but a few, including O157:H7, can cause severe illness and can lead to HUS, the Mayo Clinic said.