Gov. Chris Christie speaking on Thursday in Manahawkin, N.J. His administration has hired Randy M. Mastro, a longtime associate of Rudolph W. Giuliani, to help cooperate with a legislative investigation. Ángel Franco/The New York Times

TRENTON — As the New Jersey Assembly voted unanimously on Thursday to authorize an investigation into abuses of power by Gov. Chris Christie’s administration, Mr. Christie seemed to be maneuvering against the inquiry, hiring a high-powered defense lawyer and resisting questions about whether he would cooperate with the Legislature’s efforts.

The investigative panel created by the Assembly issued subpoenas to 17 individuals and three organizations that members believe may be able to answer why Mr. Christie’s deputy chief of staff gave the signal to close lanes onto the George Washington Bridge in September in an act of political punishment.

Mr. Christie’s administration announced that it had hired Randy M. Mastro, a longtime associate of former Mayor Rudolph W. Giuliani of New York City, to conduct an internal review and cooperate with the investigation announced last week by the United States attorney’s office into the lane closings, which gridlocked emergency vehicles, commuters and school buses in Fort Lee, N.J., for four days.

But while the statement said that Mr. Mastro would also assist with “other appropriate inquiries,” it left unsaid whether Mr. Christie intended to cooperate with the Legislature. When asked directly whether the governor considered the legislative inquiries appropriate, a spokesman for the governor, Colin Reed, declined to answer.

The day’s events underscored the sudden reversal of fortune for the governor, who just more than a week ago was widely considered the leading candidate for the Republican nomination for president in 2016.

While the governor’s political aides have often described his strategy as “governing on the offensive,” his hiring of Mr. Mastro suggested that he was on the defensive.

Members of the special Assembly investigative panel declined to say who was subpoenaed until the papers had been received, but they had said earlier that they expected the list to include members of the governor’s State House and campaign staffs whose names were in documents revealed last week, outlining the cooperation between Mr. Christie’s staff, his campaign and his operatives at the Port Authority to close the lanes and then cover up the political motive.

Mr. Christie and Mr. Giuliani, both former United States attorneys, are longtime friends — Mr. Giuliani has been among Mr. Christie’s most aggressive defenders as the bridge scandal has unfolded — and they have shared staff members and advisers, including Bill Stepien, Mr. Christie’s two-time campaign manager. (The governor cut ties with Mr. Stepien last week after documents linked him to a cover-up of the lane closings.)

Mr. Mastro, whom the statement characterized as an “outside, third-party perspective,” was an assistant United States attorney under Mr. Giuliani in New York, and later served as a deputy mayor. He now works for Gibson, Dunn & Crutcher, which has long served as Mr. Christie’s private counsel.

At an appearance on the Jersey Shore — an event postponed from last week, when documents revealed that a top aide had given the signal to close the lanes — Mr. Christie seemed somber, replacing his usual bluster with something closer to humility as he announced a new round of money for Hurricane Sandy rebuilding.

Comparing himself to the mayors of towns struck by the storm, Mr. Christie joked that he could relate to those who ran for office “thinking it was going to be one thing and then wind up having a job that turns out to be something totally different.”

He added: “When you take that oath, that tagline at the end is not, ‘If everything goes the way it’s supposed to.’ The tagline at the end is, ‘So help me God.’ All right? And I think all of us have thought about that.” 

“No one, I can assure you, ever told me or anybody on my team that it was going to be easy,” he said. “But I want to assure the people of New Jersey of one thing. I was born here, I was raised here, I’m raising my family here. And this is where I intend to spend the rest of my life. And whatever tests they put in front of me, I will meet those tests because I’m doing it on your behalf.”

Mr. Christie did not address the scandal directly, but acknowledged in passing the storm around him. “I suspect there are a few more cameras here today than we might have originally thought for a Sandy event in Manahawkin, N.J.,” he said.

Others likely to be served with subpoenas included Bridget Anne Kelly, the deputy chief of staff who gave the order, and Mr. Stepien.

The State Senate on Thursday also authorized a committee to investigate the lane closings, and said that among the people it planned to subpoena would be David Samson, whom Mr. Christie appointed chairman at the Port Authority of New York and New Jersey, which controls the bridge, and Regina Egea, who oversaw the authority from Mr. Christie’s office, and was recently chosen to become the governor’s chief of staff. Those subpoenaed will most likely have 10 to 14 days to respond.

Republicans in the Assembly joined Democrats in the unanimous vote to authorize the investigation. But that unity crumbled as soon as the investigative committee met, with Republicans arguing that Democrats, who control the Legislature, were trying to hijack the investigation for partisan purposes.

They questioned the cost of hiring the special counsel who will assist the investigation, Reid Schar, a former assistant United States attorney from Illinois who assisted in the prosecution of former Gov. Rod Blagojevich. (Mr. Schar will be paid $350 an hour, far lower than his firm would normally charge.)

John S. Wisniewski, the chairman of the Assembly panel, said he had no intention “right now” of subpoenaing the governor, and noted that no documents had linked Mr. Christie directly to the lane closings.

He said he had no reason to believe that Mr. Christie would not cooperate.

Correction: January 16, 2014

Because of an editing error, an earlier version of this article misstated the day the emails about the lane closings were released to the public. They were released on Wednesday, Jan. 8, 2014, not Thursday, Jan. 9, 2014.