US Census Bureau more than $3 billion over budget, according to gov't watchdog The GAO added the 2020 Census to its "High Risk List."

The U.S. Census Bureau’s effort to secure its computer networks from cyber attacks and hacking is behind schedule and billions of dollars over budget in preparing for the upcoming 2020 population count, a new federal report has found.

Auditors from the Government Accountability Office found the bureau is already more than $3 billion over its $12.5 billion security budget and officials could not say how much more money will be needed.

The GAO, the independent investigative arm of Congress, has been reviewing costs associated with the Census at the request of congressional leaders.

"Cost information that is accurately reported and clearly communicated is necessary to help ensure that Congress and the public have confidence that taxpayer funds are being spent in an appropriate manner," said the GAO, in a report issued August 30.

The GAO has added the 2020 Census to its "High Risk List," which tracks programs that need continued attention.

The Census Bureau faces challenges in "securing systems and data, such as developing policies and procedures to minimize the threat of phishing aimed at stealing personal information," according to the GAO.

The federal government, as mandated in the Constitution, must conduct a census every 10 years in order to count every person living in America. The data collected is then used to determine a wide range of things, including the number of U.S. House of Representatives members each state gets and how much federal funding local communities receive from the government.

For the upcoming 2020 Census, the bureau said it is upgrading its technology system to make it easier for people to respond anywhere, anytime.

Residents will be able to answer the survey online and on mobile devices, the bureau said.

The information technology costs alone are already 40 percent over budget, the GAO reported.

In October 2015, the Census Bureau estimated that the upgrades would cost $3.41 billion. By December 2017, IT costs stood at $4.97 billion. GAO auditors said they are still reviewing the extent to which the bureau’s December 2017 cost estimate is reliable.

"The amount of cost growth since the October 2015 estimate raises questions as to whether the bureau has a complete understanding of the IT costs associated with the 2020 program," the GAO said.

The Census Bureau declined to comment on the GAO’s findings, when contacted by ABC News.

"We have no substantive disagreements with the findings in this report," Census officials said in a statement responding to the GAO’s report.

“This report illustrates that significant challenges remain ahead of the 2020 Census," said Drew Pusateri, a spokesman for Sen. Claire McCaskill, who is the ranking member of the Committee on Homeland Security and Governmental Affairs and one of the congressional leaders who requested the GAO report.

In order to make sure that the systems are working properly for the 2020 Census, the bureau has been conducting a "2018 End-to-End Test," which began August 2017 and is scheduled to go until April 2019.

While the bureau has completed security assessments for 33 of the 44 systems needed for the 2018 test, there are still delays in system development, hampering the testing process, according to the GAO.

GAO auditors are concerned that the Census Bureau might not have enough time to properly test its computer networks prior to the start of counting, on April 1, 2020, creating a risk that personal data of Americans might not be fully secure.

The key is "ensuring that individuals gain only limited and appropriate access to the 2020 Census data, including personally identifiable information, such as name, personal address, and date of birth," officials said.

The delays are being caused, at least in part, by the Census Bureau’s heavy reliance on outside contractors and that the agency does not have enough staff to properly monitor them, the GAO reported.

"The Bureau is managing the integration contractor through a government program management office, but this office is still filling vacancies," according to the GAO.

As of June, 33 of the Bureau’s 58 federal employee positions were still vacant.

The GAO report does not provide reasons for the staffing issues, but auditors warned the bureau of this risk last year.

In May 2017, the director of the Census Bureau announced his retirement after more than 27 years at the agency.