Several weeks before a 22-year-old Fairfield man was arrested on serious weapons charges after triggering a lockdown and manhunt at the University of New Haven last week, he had visited Fairfield Woods Middle School, according to school officials.

In a letter sent Friday to parents of Fairfield Woods students, Principal Gary Rosato said that William Dong of Stratfield Road visited the school -- which he had attended -- about a month ago after classes were done for the day and spoke with a staff member.

Rosato said Dong left the campus "without incident," but he decided to contact Fairfield police to tell them about the visit after Dong was arrested last week at UNH.

Fairfield police are working with authorities in West Haven to investigate Dong, who was charged with illegal possession of an assault weapon, illegal possession of a weapon in a motor vehicle and breach of peace after police said he brought loaded pistols and an assault rifle to the UNH campus.

Dong lives with his family at 1336 Stratfield Road, and local police assisted West Haven police when they searched the family's home last Tuesday night.

Dong, who attended middle-school classes at Fairfield Woods through 2005, then went on to Fairfield Warde High School and graduated in 2009.

Rosato wrote in his letter that many former students make return visits to the school.

In an interview Saturday, Rosato said Dong met with a teacher during his visit a few weeks ago. Rosato declined to identify the teacher, but the principal said that Dong left on his own and was not asked to leave.

"Students come back from time to time to see their teachers," Rosato said. "That's not unusual. It certainly didn't seem unusual at the time."

Rosato said he was told of Dong's visit Thursday night, and thought local police should be aware of it in light of the incident at UNH.

Visitors to Fairfield Woods Middle School have to enter through the front office during school hours, and the principal said that after-school visitors should also go through the office. The office is staffed until 5 p.m., but there are events at the building after those hours.

Rosato said the school district frequently communicates with local police about changes it should make to keep the schools safe. Fairfield police said Saturday that they'll be stepping up patrols around local schools in light of the approaching first-year anniversary of the mass shooting at Sandy Hook Elementary School, as well as recent public-safety incidents in town.

"I think school security is obviously the No. 1 priority at this stage," Rosato said. "These are community buildings and we want them to be as safe as possible."

The revelation that Dong visited his old school comes just a few weeks after the report on the Sandy Hook massacre revealed that shooter Adam Lanza had visited that school's campus before going on his deadly rampage nearly one year ago.

Police Chief Gary MacNamara said Saturday that Fairfield Woods had contacted his department about Dong's visit only after his arrest in West Haven, so there was no apparent cause for alarm at the time of his visit. But police are investigating his visit nonetheless.

MacNamara said police understand that parents might be concerned, given the charges against Dong, but he added that so far police don't have any indication that Dong intended to do anything dangerous at Fairfield Woods.

"At the same time, it's alarming to parents, and that's why we're actively investigating the circumstances surrounding it," MacNamara said.

Police in West Haven did not immediately return a phone call Saturday seeking comment on how Dong's visit to Fairfield Woods might affect their investigation.

Fairfield PTA President Lisa Havey said that she had not seen Rosato's letter. She said school staff likely couldn't have predicted that Dong would be involved in an incident several weeks later. There were no indications of trouble during his visit, raising a question about what kind of scrutiny former students should be subject to when they return to campus.

Havey said she routinely goes back to her old elementary school, Holland Hill School, to visit and volunteer.

"Hindsight is perfect," she said. "People come back and visit all the time. You can't control what someone is going to do. But I'm really glad that there's a process in place with the school, so that if something happens, they have a plan. I'm comfortable with that."

She also praised Fairfield Woods officials for communicating with police about Dong's visit.

A report on the Sandy Hook shooting released by state investigators late last month said that investigators had analyzed global position system data and found that Adam Lanza had gone on a scouting trip of several local elementary schools just before the shooting last December. That trip included Sandy Hook, where he was once a student.