The sound of gunfire outside the Capitol forced at least five senators in the vicinity to take cover on their stomachs and led to a temporary lockdown of members of Congress and their staffs. The House and Senate adjourned for the day shortly after the incident as the shutdown extended into a third day.

Along with Senator Mitch McConnell of Kentucky, the Senate Republican leader, Mr. Boehner has long dismissed the idea that Congress would not act to prevent a damaging default, and President Obama on Thursday called a default “the height of irresponsibility.” But the failure of the House and Senate to reach a deal ahead of the shutdown has raised questions of whether Republicans could be persuaded to join in raising the debt limit before the Treasury Department runs out of money about the middle of October.

His comments were read by members of both parties as renewing his determination on the default and came as the Treasury warned that an impasse over raising the debt limit might prove catastrophic and potentially result “in a financial crisis and recession that could echo the events of 2008 or worse.”

Lawmakers said that in recent days, Mr. Boehner, who is under fierce attack from Democrats over his handling of the shutdown, has made clear that he is willing to use a combination of Republican and Democratic votes on the debt limit if need be. Representative Leonard Lance of New Jersey, one of the moderate Republicans who met privately with Mr. Boehner on Wednesday, would not provide details of the meeting, but said, “The speaker of the House does not want to default on the debt on the United States, and I believe he believes in Congress as an institution, and I certainly believe he is working for the best interests of the American people.”

One lawmaker, who spoke on the condition of anonymity, said Mr. Boehner suggested he would be willing to violate the so-called Hastert Rule to pass a debt-limit increase. The informal rule refers to a policy of not bringing to the floor any measure that does not have a majority of Republican votes.