Critically injured Secret Service agent Garrett FitzGerald — unable to move or feel anything below his shoulders since an on-duty car accident in New Hampshire last week — is determined to serve his nation again, and the agency’s chief has promised he will, the agent’s mother and fiancee told the Herald.

“If he could go back to work right now, he would,” said fiancee Joan Lyall, 28, a speech pathologist at Boston Children’s Hospital. “It’s been a dream of his to be a federal agent. In whatever capacity he can, he’ll be ready to go back.”

FitzGerald, 30, was one of four agents the Secret Service said was part of Hillary Clinton’s security detail, who were injured Dec. 29 when a car crossed over double-yellow lines on Route 16 and plowed into their vehicle head-on.

Lyall got the call as she was making dinner in their Back Bay apartment, which they share with their Newfoundland retriever, Hunny.

“I love you, you’re the best thing that’s ever happened to me,” he told her. Someone at his end had to hold the phone to his ear.

“There was a lot of confusion,” Lyall said. “But I knew he was having trouble feeling his legs, I knew he was scared, and I knew I had to get to him.”

She drove toward Frisbie Memorial Hospital in Rochester, N.H., not sure what she would find. They have been together nine years.

“It was his eyes and his smile that drew me to him,” she said. “When I walked into the hospital room and he still had those eyes and that smile, I knew we were going to get through this.”

FitzGerald’s long-term prognosis — whether he will recover sensation and movement in his limbs — is unclear. He is due to start rehabilitation in the coming weeks. But, his mother and fiancee said, he intends to remain a Secret Service agent.

FitzGerald has been an agent for a year. In 2011, he was working as an engineer for Boeing in Philadelphia when he decided he wanted to serve a greater purpose.

“He said, ‘Don’t be mad, mom. But I don’t want to be an engineer anymore,’ ” his mother, Adrienne Andresen said. “He told me he wanted to make a difference.”

He commuted to Washington, D.C., twice a week to get his master’s degree in computer forensics, while still working full time at Boeing, until he finished school and finally was accepted by the Secret Service in December 2014.

Andresen said Secret Service chief Joseph Clancy came to her son’s bedside two days after the accident and promised that he’d always have a job.

“Garrett looked at him and said, ‘Sir, I just want to come back to work,’ ” she said. “He has a drive I’ve never seen in anyone else.”

Clancy has not commented publicly, except to confirm the accident and issue a statement, “Please join us as we keep all the victims of this accident and their families in our thoughts and prayers.”

Everything has changed now for FitzGerald and his fiancee.

“The wedding was to be March 4,” said Joan’s mother, Julie Lyall. “Now we are going to march forth, however we can.”

She said, “It’s not the same story they thought they were going to have, but regardless, it’s probably going to be even more inspirational. They’re just going to prove that nothing can get in their way.”

Although the U.S. government will cover many of FitzGerald’s costs, there are changes that will come with a hefty price tag — like modifying, or perhaps moving out of, the couple’s small walk-up loft.

Rob Freed — who got married with FitzGerald as his best man just last month — is one of two friends who have set up a GoFundMe page to help cover those costs.

“It just kills me,” said Freed, a radiologist in Pittsburgh. “You see this stuff on the news, and you never think it would affect your best friend.”

To donate, go to: www.gofundme.com/garrettstrong, and tweet #GarrettStrong.