TORONTO -- It was standard practice for an Amber Alert to expire even though the five Ontario children whose disappearance prompted it had not been found, police say.

The alert was issued Tuesday afternoon, for five children from Niagara Region who had allegedly been abducted by their father.

Police said late Tuesday night that although the children had not been found at that point, the alert had expired. The expiry meant, among other things, that emergency messages were no longer displayed on highway signs and radio and television channels. This prompted widespread confusion on social media, with many people questioning why the alert was not being continued.

"How could an Amber Alert 'expire' if the children or the person responsible for taking them haven't been located?" one Twitter user asked.

Although the expiry was unusual, police say it was part of the system working as intended, as alerts in Ontario typically expire after five hours or less – although in most cases, the children are found within the five-hour window.

The exact expiry time depends on the circumstances of the case. If an alert is issued late at night, it may be extended into the following morning to ensure people who were asleep see it when they wake up.

"The Amber Alert completed its goal in notifying the community and bringing attention to the matter," Niagara Regional Police spokesperson Const. Phil Gavin told via email Wednesday.

"It is then the responsibility of the respective police service to continue to raise the alarm and put out messaging."

After an alert expires, a new alert could still be issued for the same case if significant new information comes to light.

Chris Lewis, CTV's public safety analyst and a former commissioner of the Ontario Provincial Police, questioned the reasoning behind having a five-hour expiry period on Amber Alerts.

"The reality is, if the kids are still in danger, then why wouldn't the focus, the media, the police … continue?" he said in an interview with via telephone on Wednesday.

"At five hours and one minute, are they less in danger than they were before?"

Police have said that the expiry of the alert did not affect how they conducted their investigation.

As for why the alert was issued Tuesday when the children had not been seen for at least a week – police believe the alleged abduction happened sometime between Sept. 19 and Sept. 25 – Gavin said the case had only come to the attention of police on Monday, when a missing persons' report was filed.

"As a result of the subsequent investigation by our child abuse unit there was a concern for the welfare and safety of the children," Gavin said.

Other Twitter users took issue with an Amber Alert being issued for children believed to be with their father. Parental abductions are typically not covered by Amber Alerts in Ontario, unless police have a reason to believe a child's life may be in danger.

"There has to be some fear of danger, or the public becomes complacent and they think 'Oh, here's another case where the dad didn't bring his kids home in time,'" Lewis said.

"They want people to realize 'OK, this only happens in the most dire of circumstances, and so we should be watching for this vehicle.'"

Police said Wednesday afternoon that the children whose disappearance prompted the alert had been located and were safe.