Attacks on the online voting system used to select the new leader of the NDP slowed the process down by several hours Saturday, frustrating members and forcing the party to go well past the deadline they'd hoped to meet.

Initial slowness on the first round of voting eventually turned into a total shutdown as officials scrambled to figure out what the problem was. Lines snaked around the voting booths at a Toronto convention centre and a Twitter stream dedicated to the leadership race filled with complaints.

The party first chalked it up to the servers being overwhelmed, but with a small percentage of members actually casting ballots the day of the leadership convention, they were soon facing questions over a possible denial of service attack, in which an attacker tries to overwhelm a server with requests.

Senior party official Brad Lavigne said someone was deliberately trying to mess with their system.

"The only thing they were able to achieve was a little delay," Lavigne said. "There is someone outside the system who is attempting to mess up our system."

The system was not hacked, Lavigne said, and the integrity of votes cast was not compromised. But the attack delayed third- and fourth-round voting, officials said.

Less than half of eligible members voted

The low voter turnout was another looming issue. More than 58,000 members voted in advance, leaving about 11,000 people voting Saturday, online or at terminals set up at a Toronto convention centre. About 131,000 New Democrats were eligible to vote online, with their smart phones or in person. Approximately 4,200 of those members went to the convention.

The results of the fourth and final ballot were finally announced just after 9 p.m. ET, with Montreal MP Thomas Mulcair taking over the role left by Jack Layton after he died of cancer last August. The results had been delayed by an extra hour at the last minute, the last of a number of delays because of the online attack.

Officials say they may be able to identify the source of the attack, which jammed the online voting system, provided by Spanish company Scytl.

Party president Rebecca Blaikie told Radio-Canada that they isolated two IP addresses behind the attacks, which at one point sparked more traffic than if all 131,000 members were trying to vote at same time. An IP address is a number assigned to an internet connection that may be able to point the party to the perpetrator.

A party spokeswoman initially said the servers were overloaded.

"The number of people visiting the site is large," Sally Housser said earlier Saturday. "We're trying to identify the problem now."

Canadian Heritage Minister James Moore, who observed the convention for the Conservatives, called the process disappointing. Online voting systems not infallible, he said.

"It's pretty risky if you're a national party and you're trying to present yourselves as an alternative to government and you can't organize a vote. I think that's a challenge," Moore said.

Housser said there was a little bit of a delay in the first round of voting that ended at 9 a.m. ET Saturday. For the third round, the computers at the convention centre kept freezing, so they were shut down to make corrections possible, she said.

Voting delayed throughout the day

On the second round of voting, which started at 11 a.m., the party had to extend voting twice to accommodate the technical issues and released the results just before 2 p.m. ET.

The third round delays meant the results of that ballot weren't released until after 6 p.m., when the attention of many Canadians turns to NHL hockey. Senior NDP officials had said they hoped to have a winner before Saturday night's games.

Members started reporting problems after the second round opened at 11 a.m. ET, tweeting their frustrations.

Housser said the party had sent an email to members to tell them of the delay and that they had people available to answer technical questions by phone.

Asked whether they'll refer the matter to Elections Canada to investigate, officials said there's no reason for that because it's a party issue.