Statistics Canada's online system crashed the first day it began to collect census data from Canadians last month because of design flaws, not because too many people were using it simultaneously as the agency initially suggested.

Internal documents also indicate the system was broken for about three hours on May 2, longer than the 45 minutes the agency initially claimed, and affected "millions" of Canadians trying to file.

Statistics Canada claimed in a tweet that evening that because of the "high numbers" of Canadians filing their census information, the website was "overwhelmed by the enthusiasm."

Thanks Canadians for responding in such high numbers. The site was overwhelmed by the enthusiasm, but we are back! <a href="https://t.co/SnxAmqTRSr">https://t.co/SnxAmqTRSr</a> —@StatCan_eng

In fact, traffic on the website was only at about 50 per cent of expected capacity, according to documents obtained by CBC News under the Access to Information Act.

The trouble began at 6:59 p.m. ET on May 2, when the website experienced congestion for 35 minutes, preventing some from filing their data.

The documents blame the digital traffic jam on the large size of Statistics Canada's so-called "landing page," the first page users see when signing on. The page design used a lot of bandwidth, and clogged the system as it was being downloaded to thousands of home computers.

Bandwidth saturation

"Client reported bandwidth saturation [100 per cent utilization] with less than 50 per cent of the expected concurrent users connected," says a Shared Services Canada report.

Caught off guard, Statistics Canada quickly reduced the size of its landing page and operations were restored to normal more than a half-hour later. Meanwhile, an unknown number of Canadians were unable to file their information as the system froze them out.

Two more technical issues hit the troubled system that night.

At 8:45 p.m. ET, there was an internal glitch in the way the web server communicated with the Statistics Canada database, forcing a complete shutdown of the system for half an hour while technicians repaired the problem.

Scheduled maintenance — please try again later. - Misleading notice posted May 2 on Statistics Canada's census website

Every Canadian trying to file online was then shut out, and a highly misleading notice was posted online to explain the problem: "Scheduled maintenance — The online questionnaire is currently undergoing maintenance. Please try again later."

A Shared Services Canada report says this second glitch had "never been seen" in the development, testing and quality assurance checks before the website went live.

There was also a third incident that night — but details are completely blacked out in the released package. The documents cite a section of the Access to Information Act that allows censorship on the basis that making the information public could "facilitate the commission of an offence" by exposing a "vulnerability" in the computer systems.

Not fully resolved

A Shared Services Canada report says the problems that began at 6:59 p.m. that evening were not fully resolved until 9:55 p.m., or almost three hours later.

Minister of Innovation, Science and Economic Development Navdeep Bains announced the reinstatement of the long-form census Nov. 5, 2015. (CBC)

"Census users trying to submit their information are unable to do so, impacting several million households," says the report.

At the time of the incidents, Statistics Canada rebuffed a series of questions from CBC News. A spokeswoman simply repeated the claim the website was down for only 45 minutes, and said the agency was "following up to determine the cause of the interruption."

"We thank Canadians for their enthusiasm in completing their census online," she added.

Neither Statistics Canada nor Shared Services Canada responded immediately to new questions arising from the documents released to CBC News.

StatsCan has said more than five million census questionnaires were received by May 10, the official census day, most of them filed online.

Letters from Statistics Canada containing access codes for the 2016 census ask recipients to 'please complete it by May 10.' When its website crashed on May 2, the agency put up a misleading notice referring to 'scheduled maintenance.' (Alistair Steele/CBC)

Some 15 million households were sent census notices through Canada Post in early May, with most receiving a 16-digit access code rather than a questionnaire to encourage them to file online. May 2 was the first day they could do so on the website.

On May 5, the agency suspended census collection activities in the Fort McMurray, Alta., area because of the devastating wildfires.

The budget for the 2016 census was about $715 million spread over seven years of planning and operations.

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