A 36-hour cross-border police investigation has thwarted an alleged plot by four people to open fire in a public venue in Halifax, RCMP said Friday night.

A 19-year-old man found dead in a Halifax-area home early Friday morning was one of four people who police allege planned to kill themselves after a public attack on Feb. 14.

Police arrested three other suspects on Friday morning, and say the threat has been averted.

A 20-year-old Halifax man and a 23-year-old woman from Geneva, Ill. were arrested without incident at Halifax Stanfield International Airport at approximately 2 a.m. on Friday morning, police said.

Shortly after 11 a.m. on Friday, police arrested a 17-year-old Cole Harbour, N.S. man at a Cole Harbour residence.

In a news conference on Friday evening, Nova Scotia RCMP Commanding Officer Brian Brennan said the threat has been eliminated, but asked the public to remain vigilant.

Brennan thanked the Geneva Police Department in Illinois and others for helping in the investigation that began on Thursday morning when police first learned of the threat.

RCMP and Halifax Regional Police then launched an investigation, which led them to a home on Tiger Maple Drive on Thursday evening.

The RCMP’s Emergency Response Team entered the home at about 1:20 a.m. and found the 19-year-old man dead.

Police say he was the only person in the house at the time.

Nova Scotia’s Serious Incident Response Team has taken over the investigation into the man’s “sudden death”, Brennan said.

RCMP continue to lead the original investigation into the weapons-related threat.

The three suspects remain in custody, their role in the plot having yet to be determined, Brennan said.