A state trooper who was guarding the home of 9-year-old Charlotte Sena has come under scrutiny for failing to arrest Craig Ross Jr. when he allegedly dropped off a ransom note, according to a report.

The 47-year-old suspect drove away in his pickup truck before dawn Monday without being stopped by the trooper, multiple sources familiar with the matter told the Albany Times-Union.

There has been no allegation that the trooper mishandled the incident, but New York State Police is investigating whether Ross should have been arrested at the time, the sources told the news outlet.

On Tuesday, Gov. Kathy Hochul and officials with the agency declined to discuss the trooper’s actions.

State Police and the governor’s office said they would not comment due to an “ongoing investigation,” the Times-Union reported.

Meanwhile, the news outlet cited another source briefed on the investigation as saying that Ross may have forced Charlotte to write the ransom note for $50,000 herself.

Hochul told reporters earlier that State Police had been stationed outside the home as the family remained at the campground where Charlotte was last seen overnight Sunday.

“(At) 4:20 a.m. (Monday), the car pulls up to a mailbox, something is left,” she said, according to the newspaper.

“State Police immediately go to the mailbox and identify what is a ransom note that had been left behind for (the missing girl),” Hochul said.

“State Police worked diligently trying to find a match for a fingerprint. The first one they tried wasn’t successful. Second one was to identify any other prints in the New York State database that would be a match,” she added.

She said later that day that a fingerprint on the note was matched to Ross from a 1999 arrest for driving while intoxicated in Saratoga Springs.

But neither ​Hochul nor State Police officials explained why the trooper at the house didn’t stop the suspect after observing him putting a note in the mailbox.

The missive would have provided enough evidence to immediately detain Ross, police sources told the Times-Union.

The suspect was arrested after a brief struggle with cops while in his underwear about 14 hours after he dropped off the note, the outlet reported.

Charlotte, who was found inside a cupboard in the camper, was taken by ambulance to Albany Medical Center Hospital.

Ross has been charged with first-degree kidnapping and is being held in Saratoga County jail as authorities continue their investigation.