Frank James was quiet and gruff, his neighbors in Milwaukee said.

A neighbor of Frank R. James, whom the New York Police Department has called a person of interest in Tuesday’s mass shooting on a subway train in Brooklyn, said in an interview that Mr. James was gruff and standoffish, and once confronted her over a key left in her apartment door.

The neighbor, Keilah Miller, said she had not seen Mr. James since late March — a timeline that aligns with Mr. James’s description of his recent travels, posted in a YouTube video last month.

Ms. Miller, 32, said that she lived in an adjacent unit to Mr. James, 62, and that he had moved into the two-story triplex in Milwaukee within the past year.

Ms. Miller said she had heard him yelling on the phone several times, including a conversation in which he complained about ignorant people. After she mistakenly left her key in her lock, they had an altercation in which she recalled him yelling, “Don’t ever do that again!”

He walked down the street almost every morning, she said, but never said hello.

“Like if I say, ‘Hello, good morning,’ he just grunts at me like he’s some old grumpy Black man,” said Ms. Miller, who is Black.

“He is a really weird neighbor,” she added.

A friend who mistakenly went inside Mr. James’s apartment at one point, she said, described it as “dirty and messy.” Ms. Miller said she had never seen anybody else with him.

When Ms. Miller returned home on Tuesday evening, there was a television station outside the chain-link fence surrounding her building, but no noticeable law enforcement presence.

“They should probably go in there and sweep it with the police because this is terrifying now,” she said. “I’m a little scared and worried.”

One former neighbor from an apartment in Milwaukee that Mr. James previously lived in said Mr. James was quiet and walked with a limp, and that it was a surprise he would be connected with the shooting.

The neighbor, Mike Lopez, 38, said he never spoke to Mr. James but often saw him pushing a small cart with groceries or other possessions.

“I didn’t see him as no threat or nothing,” Mr. Lopez said. “I mean, I don’t see him as capable as that. He can’t move like that, man. He wasn’t fast.”

The police have not said whether they believe that Mr. James was the gunman, and did not call him a suspect. But the key to a U-Haul van he had rented was found among the gunman’s belongings at the scene of the shooting, the police said.

In a video posted on YouTube, Mr. James described his plan to pack up his apartment in Milwaukee and drive a rented U-Haul to Philadelphia, where he said he had rented a place to stay.

In the video, he describes his concern about returning to Philadelphia, which he refers to as “the danger zone.”

“On the drive I’m just thinking because I’m heading back into the danger zone, so to speak, and it’s triggering a lot of negative thoughts,” he said, “because I do suffer from — have a bad case of post-traumatic stress from all the things I’ve been through.”

Ashley Southall contributed reporting.