Michigan Gov. Rick Snyder (R) and EPA Administrator Gina McCarthy testified in front of the House Oversight Committee on March 17, and things got a little heated. (Peter Stevenson/The Washington Post)

The head of the Environmental Protection Agency conceded Thursday that her agency was too slow to intervene in the Flint, Mich., water-contamination crisis and less forceful than it should have been when federal officials told a recalcitrant state bureaucracy to act.

Despite learning last June that three homes had lead-tainted water and expressing her concern over the situation in a September email to top staffers, Administrator Gina McCarthy did not use her emergency powers until late January. Assuming the state would make good on promises to take decisive measures, the EPA did not push Michigan’s environmental quality agency hard enough to begin treating the water, McCarthy acknowledged.

But she stood up to often-furious questioning at a congressional hearing that included Republican calls for her resignation, asserting that under the law her agency had done all it could to protect Flint’s 95,000 residents. She refused several times to accept blame for the catastrophe, laying the responsibility on the witness seated next to her, Republican Gov. Rick Snyder.

State officials “slow-walked everything they needed to do. That precluded us from doing what we had to do,” she told the House Oversight and Government Reform Committee. “We were strong-armed. We were misled. We were kept at arm’s length. We couldn’t do our jobs effectively.”

Flint in crisis: Tainted water, little hope

Snyder adopted a more conciliatory tone as several Democrats called on him to quit, admitting culpability and noting that he had dismissed several state officials. But he bluntly suggested that the EPA had failed in its oversight role and its obligation to warn the public.

Snyder had little success fending off questions about why his staff knew how dire the situation had become but he did not.

“It looks like everyone knew about these problems except you,” said Rep. Elijah E. Cummings (Md.), the committee’s ranking Democrat. “You were missing in action.”

The long-anticipated hearing provided the highest-level jousting yet over a public health disaster that has revealed another partisan divide on Capitol Hill and the campaign trail.

Democratic presidential candidates Bernie Sanders and Hillary Clinton have called on Snyder to resign, which some committee Democrats also urged Thursday. But Chairman Jason Chaffetz (R-Utah), a longtime critic of the EPA, joined other Republicans in targeting the federal agency during officials’ testimony in the two hearings this week.

Flint’s water was poisoned when the city, under the control of a state-appointed emergency manager, switched to the Flint River for its supply in April 2014. The state Department of Environmental Quality failed to ensure that anti-corrosive chemicals were added to the water, which leached lead from aging pipes and sent it through the taps to consumers.

At Snyder’s order, the city switched back to Lake Huron water last October. But anyone who drank the lead-tainted water was exposed to the toxin, which can cause learning disabilities, behavior problems and illness. Children younger than 6 are especially vulnerable.

People still cannot drink the water unfiltered. Nine have died from outbreaks of Legionnaires’ disease that may be linked to the water supply.

Hundreds of residents traveled to the Capitol to watch Thursday’s proceedings, lining up for seats long before the hearing began at 9 a.m. In one overflow room, the crowd held more anger for Snyder than for McCarthy, hooting at many of his remarks and repeatedly demanding that he be jailed.

Little new ground was covered. Both witnesses staked out their positions from their opening statements and defended that turf for the better part of four hours.

For McCarthy, that meant explaining that the EPA was working hard on the issue last summer and fall even if it did not have the data and evidence of state foot-dragging that the agency needed to move in and take control. It offered Michigan its experts on anti-corrosion and water chemistry but was rebuffed, McCarthy said.

In October, the EPA established a task force to provide technical expertise to the state and city. In November, the agency launched an audit of how well Michigan carries out the requirements of the Safe Drinking Water Act. Susan Hedman, the EPA’s former Midwest chief who resigned in January, took on state officials who called an agency whistleblower “a rogue employee,” McCarthy recounted.

That did her little good with Chaffetz.

“If you want to do the courageous thing, like you said Susan Hedman did, you too should resign,” Chaffetz thundered.

When McCarthy tried to explain the limits of the EPA’s power under the law, Chaffetz repeatedly cut her off. “Well, it failed,” he yelled. “You failed.”

Several times, Snyder mentioned his guilt over what he might have done to forestall the crisis if he had received better advice from the experts in his own bureaucracy. “Not a day or night goes by that this tragedy doesn’t weigh on my mind,” he said in his opening remarks. “The questions I should have asked. The answers I should have demanded. How I could have prevented this.”

Rep. Matthew Cartwright (D-Pa.) was having none of it.

“Plausible deniability only works when it’s plausible,” he said angrily. “You were not in a medically induced coma for a year.

“We’ve had enough of your false contrition and your phony apologies,” Cartwright continued. “You’re doing your dead-level best to spread accountability. . . . You need to resign, too.”

Noting the Legionnaires’ disease outbreak, Rep. Brendan Boyle (D-Pa.) asked Snyder: “As the governor of an administration that failed and poisoned its own people, don’t you have a moral responsibility to resign?”

Amber Phillips contributed to this report.