The US Department of Labor has launched an investigation into reports that some of the numbers in the government’s monthly jobs report – one of the world’s closest watched economic indicators – may have been fabricated in the run-up to the 2012 election.

According to The New York Post, the issues center around the politically charged release of job figures towards end the 2012 presidential campaign. From August to September that year, the unemployment rate fell sharply, from 8.1% to 7.8%.

The new rate was a 44-month low, the lowest level since Barack Obama first took office, and a major fillip to his re-election campaign. The numbers, according to the Post, were manipulated.

The Department of Labor, which is responsible for publishing unemployment data, confirmed to the Guardian that an investigation into the affair was now under way. "We have contacted the Census Bureau about this, and it has been referred to the Commerce Department inspector general," said spokesman Steve Barr.

He said it was "premature at this stage" to discuss whether the Bureau of Labor Statistics would have to restate jobs numbers for the period in question.

A spokeswoman for the Census Bureau was not immediately available for comment.

The dramatic drop last year attracted some skepticism at the time. Jack Welch, former boss of GE, called the fall “unbelievable” and accused the White House of cooking the books. “These Chicago guys will do anything … can’t debate, so change numbers,” he said on Twitter.

His comments were widely ridiculed. Hilda Solis, then US labour secretary, said she was “insulted” by his comments and called them “ludicrous.”

Confidential documents obtained by the Post say census employee Julius Buckmon was caught faking results for the Philadelphia region, one of six regions that used to generate the monthly unemployment poll.

Each month the Labor Department’s Bureau of Labor Statistics releases its non-farm payroll report. The report has two components. One estimates the number of new jobs created that month from payroll figures, using figures from US employers. The second part, known as the household survey, calculates the national unemployment rate using a poll of US households.

The Labor Department requires the Census Bureau to achieve a 90% success rate on its interviews. According to the Post the New York and Philadelphia regions were falling short of that target. Buckmon told the newspaper that his superiors told him to fill in the gaps for Philadelphia.

“It was a phone conversation – I forget the exact words – but it was, ‘Go ahead and fabricate it’ to make it what it was,” Buckmon told the Post.

The unemployment rate is calculated each month using interviews with 60,000 households. Each of those households represents 5,000 homes in the US. It is unclear what impact Buckmon’s interviews had on the results. According to the Post, Buckmon was never told how to answer the questions.

Gus Faucher, a senior economist at PNC Bank, said the relatively small size of the sample meant it was volatile, and small changes could have large effects. “Reading too much into one month’s figures is misleading,” he said. “The fact is the unemployment rate has been falling. That’s the larger story,” he said.