WASHINGTON — Congressional Democrats on Monday called for a hearing into “regulatory gaps” highlighted by a chemical spill that has contaminated the water supply for 300,000 West Virginia residents.

“As we begin to consider ideas to reform the Toxic Substances Control Act, it is critically important that we understand how the law allowed a potentially harmful chemical to remain virtually untested for nearly 40 years,” Rep. Henry Waxman of California, top Democrat on the House Energy and Commerce Committee, and Rep. Paul Tonko, D-N.Y., ranking member of the panel’s Environment and the Economy Subcommittee, said in a letter.

The lawmakers expressed concern about the lack of information on the chemical blamed for the contamination — 4-methylcyclohexane methanol, also known as crude MCHM.

“This incident raises an important question that merits the subcommittee’s attention,” they wrote. “Simply, why don’t we know more about the potential health effects of exposure to this chemical?”

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The ban on using tap water was lifted in some places Monday as the aftermath of the spill stretched into a fifth day.

Officials advised residents, however, that the black-licorice smell of the obscure coal-cleaning chemical was expected to linger in tap water even after the water was deemed safe to drink.

A spokeswoman for the committee’s Republican leadership said, “The committee is actively monitoring the federal investigation and working to fully obtain the facts surrounding situation.”

On the other side of the Capitol, the Senate Environment and Public Works Committee, chaired by Sen. Barbara Boxer, D-Calif., has opened an investigation into the spill, which came from a leak found last week in a storage tank along the Elk River in the Charleston area.