Federal officials are collecting data on the levels of lead in Flint's water system and hope to be able to share the findings in April, U.S. Environmental Protection Agency Administrator Gina McCarthy said.

"Hopefully, by next month, we'll be able to give people news — whether that is a clean bill of health or not, I don't know," McCarthy told reporters after touring a food bank Tuesday in the city. "But again, the challenge with lead will still remain, because lead is in people's homes."

Flint was under state financial control in 2014, when officials switched its water service from Detroit's system to the Flint River. But state officials did not require Flint to add anti-corrosive chemicals when the city began drawing water from the river. The water caused lead to leach from aging pipes and water lines into some homes. Tests have shown high lead levels in some Flint children.

"The numbers we're seeing are good, but I am not going to tell people anything more than that until I see the data and it's based on science," McCarthy said.

On Tuesday, the state said water tests at hundreds of homes show 91 percent were below an important benchmark for lead. The latest tests occurred at 423 sites, with 386 below 15 parts per billion for lead, Gov. Rick Snyder said. Eight samples exceeded 100 parts per billion. Home filters distributed to Flint residents are good for up to 150 parts per billion of lead.

Results announced last week from 175 sites showed 89 percent were below the lead action level.

The results from so-called sentinel sites will help authorities decide when Flint residents can resume using unfiltered water.

Beginning this week, Virginia Tech University researchers will work with Flint residents to collect water samples from more than 200 homes that first were sampled in August 2015. Virginia Tech professor Marc Edwards had helped expose the lead problem and is now assisting both the city and state.