On a day usually associated with celebration, flags will fly at half-staff on New Year's Eve at all New Jersey state offices in honor of State Trooper Lt. William Fearon, who died this week in the line of duty.

Fearon, 49, was not felled by a bullet or swerving auto, but almost certainly by toxins he inhaled after the terrorist attack that destroyed the World Trade Center in New York City on Sept. 11, 2001.

"He was a hero," his physician, Iris Udasin, said Friday.

Fearon searched for victims - later for their remains - "for weeks" after the towers collapsed, and "on and off for the next two years," said Udasin, director of the World Trade Center Health Program at Rutgers University.

Despite assurances by Christie Whitman, a former governor of New Jersey then serving as administrator of the Environmental Protection Agency, that the air at ground zero was "safe to breathe," it was later discovered to be littered with 400 tons of the asbestos used to construct the twin towers, along with significant amounts of mercury, benzene, fiberglass, and possible radioactive materials from fire detectors.

Fearon, married with three children, was diagnosed in May 2015 with a glioblastoma, a rare, aggressive, and malignant brain tumor that can affect speech, muscle movement, reading, and thought.

He began treatment immediately after his diagnosis, Udasin said, and lived longer than many who contract glioblastoma. But the outcome is almost always fatal in patients who display symptoms before treatment, she said, because the tumor is by then so far advanced.

Udasin said that during his time at Rutgers' Biomedical Health Sciences Center in Piscataway, N.J., "Billy," as he was known, would dispense "No Fear" wristbands to patients and visitors, and "dress up in costumes" to entertain children receiving chemotherapy.

Nevertheless, she said, "the guy died of a horrible cancer. I'm glad the state is honoring him."

A 1994 graduate of the state police academy, Fearon lived in Cedar Grove, Essex County, and served his entire career with Troops B and D.

Gov. Christie ordered state and American flags to fly at half-staff outside state offices all day Saturday "in recognition of the life and in mourning of the passing memory of his service."

The program at Rutgers is one of eight World Trade Center Health Centers funded by the Centers for Disease Control and Prevention's National Institute of Occupational Safety and Health.

Their mandate is to provide "monitoring exams and treatment to the responders of the WTC attack," including those at the Pentagon and in Shanksville, Pa., and for "survivors who were present in the dust cloud . . . in the New York City disaster area."

Together they serve about 75,000 people for trauma, asthma and other respiratory ailments, gastrointestinal disorders, chronic pulmonary disease, and cancers, according to published reports.

Udasin said the Rutgers center treats about 1,500 patients on a regular basis, and 1,000 more on an occasional basis. "Over 200 have cancers," she said.

She said it is not possible to assert with certainty that Fearon's tumor was directly caused by his exposures to carcinogens at the site of the collapsed towers, in part because glioblastomas are so rare that frequencies of occurrence by age and location are hard to correlate.

Additionally, a comprehensive study by the New York Health Department found "no clear association between cancer and the debris at the World Trade Center site," according to a Reuters news agency report last year.

Published in the Journal of American Medical Association, the 2012 study observed 55,778 New York residents who were near the World Trade Center on the day of the attacks and later enrolled in the World Trade Center Health Registry.

"Among those observed, 1,187 had been diagnosed with cancer by the time of the study," Reuters reported. "When comparing this number to all New York state residents, the study found an increased risk of prostate cancer, thyroid cancer, and myeloma, but ultimately could not connect the diagnoses to 9/11 due to a lack of statistical significance."

Nevertheless, the researchers noted, "the presence of carcinogenic agents raises the possibility that exposure to the WTC environment could eventually lead to cancers."

doreilly@phillynews.com 856-779-3841