The first calls that police received about an enormous gathering at a castle-like mansion on Long Island came shortly after midnight on Monday, the authorities said. Neighbors reported a suspicious number of cars racing around and vehicles parked bumper-to-bumper on their normally quiet streets.
As more reports came in — including one from the home’s owner, who had rented his property to a guest on Airbnb and discovered the gathering while watching his security cameras from afar — the police went to the house, a lavish stone building in Brookhaven, N.Y.
When they arrived, they found as many as 400 people gathered for a party, officials said, in an explicit violation of New York State’s limits on private gatherings during the coronavirus pandemic.
The party, held at a gated luxury home owned by a one-time rapper who once hoped to be New York City’s nightlife mayor, was the latest in a string of illicit gatherings in Suffolk County on Long Island. Persuading people to obey state directives and public health guidance has proved challenging for officials, even as a surge of coronavirus cases sweeps across New York.
“There’s no question that an event like this is a public health issue,” said Chief Stuart Cameron of the Suffolk County Police Department at a news conference on Tuesday.
For months, elected leaders and officials across the United States have been pleading with residents to avoid indoor gatherings over concern that they had been contributing to an uptick in cases this fall. Though epidemiologists have been less certain that such get-togethers are the source of infections, a number of state and local governments have established restrictions to limit them.
Last month, Gov. Andrew M. Cuomo imposed a 10-person limit on all private gatherings in the state. Though a number of sheriffs said they would not enforce the order, especially on Thanksgiving, Suffolk County’s executive said his county’s police department planned to uphold it.
For months, Suffolk County, at the eastern end of Long Island, has made news for gatherings both large and small. In July, when the state had just emerged from a crushing wave of the pandemic, more than 2,150 people attended a charity concert featuring the D.J. duo the Chainsmokers. In October, county officials announced a number of parties that they called “superspreader events,” including a gathering of 200 to 300 people in Farmingville, N.Y.
The problem is not exclusive to Long Island. The New York City sheriff’s office has broken up large parties every weekend over the last several months.
“I just think there are some people who, despite all of the warnings and the damage that we’ve seen from this virus, don’t think they need to follow the rules and aren’t concerned about others,” Steve Bellone, the Suffolk County executive, said in an interview on Wednesday.
Mr. Bellone added that despite the attention paid to such large events, small gatherings had actually been driving an uptick in cases in Suffolk County since Halloween. Such get-togethers have become the predominant source of spread in the county, he said.
Over the last month, Suffolk has seen its positive test rate move from 1.2 percent to 5.2 percent, Mr. Bellone said. Hospitalizations rose from 42 on Nov. 1 to 248 on Dec. 1, and Mr. Bellone said the numbers had started to jump again after Thanksgiving.
The party in Brookhaven particularly stood out, he said, both because of its size and because it was hosted not by the homeowner, but by an out-of-state visitor who had rented the house on Airbnb.
Officials were still trying to identify the person responsible for the party, who they believed was from New Jersey, the police said. Investigators did not yet know if the person who rented the Airbnb was the person who threw the party.
A Suffolk County police spokeswoman said that charges had not yet been filed on Wednesday morning.
Officials believed the party was a “high-end” affair, with most attendees in their 30s or 40s, Mr. Bellone said. The police who reported to the scene said that many of the cars parked nearby were luxury vehicles or sports cars.
When the police arrived at the home, a 5,000-square-foot stone building situated on nearly 10 acres on a dead-end street, the party’s attendees followed orders to disperse, Chief Cameron said.
But because of the sheer number of people in attendance, the police were on the scene for close to four hours as they tried to clear every guest, he said. The department’s aviation, emergency service and canine units were all involved, he said.
Despite violating the ban on gatherings, none of the party’s attendees received summonses, the police spokeswoman said.
Officials were also not planning to file any charges against the homeowner, who Chief Cameron said had cooperated with the investigation and had offered proof that he had told the person renting the house about the gathering limit.
The owner became aware of the gathering through cameras that were installed at the house, Mr. Bellone said. One of the guests appeared to have tried to disable the cameras so the party would escape notice, he added.
An Airbnb listing for the house appeared to have been removed on Wednesday morning. But a similar listing on Tripadvisor advertised the home as an “absolutely majestic castle” with 11-foot ceilings and a 15-foot bar. The rate for a three-night stay is $1,872, and the listing notes that any parties and events must be discussed in advance.
The police did not name the home’s owner, but tax records identified him as Matthew Demar.
Mr. Demar did not immediately respond to a request for comment. According to Newsday, he was a rapper in the 1990s who used the name Kid Panic and is a former radio DJ. In 2016, he released a song in support of President Trump titled “Make America Great Again,” Newsday reported.
A video for the song, which has more than 200,000 views on YouTube, shows Mr. Demar dressed in a suit and rapping in various places around New York City. The following year, Mr. Demar appeared in a YouTube video expressing his interest in becoming New York City’s nightlife mayor, in which he called himself one of the first white rappers.
The home, at 51 Hawkins Lane, is currently listed for sale at $949,999 and has repeatedly moved on and off the market since 2014, according to Zillow. Mr. Demar purchased it in 2009, property records show.