Two whistleblowers at a pharmaceutical company responsible for one of the largest drug price increases in US history said the company bribed doctors and their staffs to increase sales, according to newly unsealed documents in federal court.

The effort, the whistleblowers said in a lawsuit against the company, was part of an intentional "multi-tiered strategy" by Questcor Pharmaceuticals, now Mallinckrodt, to boost sales of H.P. Acthar Gel, cheating the government out of millions of dollars.

The price of the drug, best known for treating a rare infant seizure disorder, has increased almost 97,000%, from $40 a vial in 2000 to nearly $39,000 today.

The Justice Department has now intervened in the case after conducting its own extensive investigation -- a sign that the government believes the allegations levied by the whistleblowers are credible. In a statement to CNN, Mallinckrodt did not deny the accusations but said the fault lies primarily with Questcor.

The bombshell allegations lay bare what the whistleblowers say was a culture designed to sell the drug at all costs, from lying to the Food and Drug Administration to offering bribes to doctors.

The price increase, combined with an aggressive sales push in rheumatoid arthritis, multiple sclerosis and other areas, has pushed the drug's annual sales over $1 billion.

Many of those sales are driven by Medicare reimbursements. A CNN investigation last year found that Medicare spending on Acthar had risen dramatically -- more than tenfold over six years -- to some $2 billion.