Younger is arrested in Canada after the FBI filed charges of facilitating terrorist activity, authorities say.
Kingston, Ontario, is faced with an additional number of consultations for a person planting an explosive with the intention of causing death or injury.
Police reported that a small airplane recently spotted flying over the city in odd hours is part of their investigation.
A second individual was arrested, but later released without charge.
The young men and the second man, an elderly man, were detained Thursday in Kingston, a town about 200km (125 miles) from the capital of Ottawa.
The authorities described the couple as friends.
According to Canadian law, the identity of the young person accused of a crime is kept confidential.
The Royal Canadian Police (RCMP) Ch Supt Michael LeSage said the forces had received the FBI signal in December.
US authorities have pointed out to their Canadian counterparts a "well-grounded and reliable attack plot", although there was no intended purpose or schedule, police said.
One home was evacuated in the early hours of the morning on Friday after the police found it to be an "explosive substance" that was blown up by a police bomb squad.
Ch Supt Lesage called Friday's "significant" charges, but said there was no imminent threat to the community. The investigation continues.
The second person was identified by the Canadian Broadcasting Corporation as a 20-year-old Syrian family.
Kingston's architect Bill Clark confirmed to the BBC that the family had come to Kingston as privately sponsored refugees in the summer of 2017.
A coalition of local churches gathered three years ago to bring a refugee family to Canada.
Bronek Korcinsky, one of the sponsors, told Reuters he is concerned about the arrests that could affect the perceptions of Muslims and refugees in Canada.
"It's a fear because it's kind of a badly informed knee reaction that puts each group in potential danger," he said.
The Federal Minister for Public Safety said the threat level in the country has remained on average for where it has been since 2014.
Canadian Public Safety Minister Ralph Goedal urged people on Friday not to draw conclusions for the case before the police finished their work.