The Zadroga Act, a federal law providing medical monitoring and treatment for 9/11 first responders, expired overnight after Congress failed to extend it.

Patients still can get their health care for now, but the program's administrators warn it will face challenges by February and will have to begin shutting down by next summer. Letting the program expire creates "enormous anxieties and fears in the minds of very sick people," said Sen. Kirsten Gillibrand (D-NY), who has been lobbying her colleagues to make the program permanent and recently was joined by comedian Jon Stewart.

New York City Mayor Bill de Blasio said it was unacceptable for Congress to let it expire. "Congress must stop putting politics ahead of our heroes' health," he said in a statement.

The Sept. 11 program is one of several that expired at midnight due to congressional inaction. While Congress moved toward passing legislation to keep government agencies open, there are some programs that depend on further action to operate long-term.

John Feal, a former World Trade Center demolition worker and leading advocate for sick responders, has pressed lawmakers to pay attention to the Sept. 11 program.

"People are dying and suffering, and Congress can easily close this wound," Feal said. "But they continue to add salt to it."

The Zadroga Act, named after a responder who died after working at ground zero, first became law in 2010 after a debate over the bill's cost. Proponents are seeking the law's permanent extension in part because some illnesses may not manifest until years later, after the statute of limitations for worker's compensation or certain state laws may have run out.

House Republicans have been supportive of the program, but have opposed its permanent extension because they say they want the chance to periodically review it and make sure it is operating soundly. The Senate has not moved a bill.

In a letter to the Senate, Centers for Disease Control and Prevention Director Tom Frieden said if the law isn't extended, the World Trade Center Health Program "will begin to face significant operational challenges" by February. By next summer, the program's 72,000 enrolled beneficiaries will have to be notified that they may not receive health care beyond September 2016, and the program will have to start to shut down. Frieden said that process could cause patients additional stress.