The prosecution is considering expanding its investigation into the previous government's highly controversial project to refurbish the country's four major rivers, sources said Sunday.

The administration of former President Lee Myung-bak carried out the 22.2 trillion won ($19.8 billion) project to refurbish the rivers to prevent floods and boost tourism. But environmental activists and the opposition party have long accused it of causing irreversible environmental damage.

Local large construction companies that participated in the massive project, including Hyundai Construction & Engineering Co., have been charged with colluding to win bids and creating large secret funds.

According to the sources, the Seoul Central District Prosecutors' Office is considering forming a joint investigative team to conduct an effective probe into the allegations.

Currently, the office's special investigative department is looking into a complaint that 12 incumbent and former executives of Hyundai Construction & Engineering misappropriated company money to create a secret fund.

Another department is looking into allegations that 16 former and current chief executives of local builders colluded with one another to win bids for the mega project.

The prosecution's move comes as the current government has vowed to conduct a thorough investigation into allegations surrounding the river restoration project.

On Tuesday, President Park Geun-hye pledged to include opposition-recommended experts, if necessary, in a government-led investigation of the project.

Early this month, the environment ministry said the government will push to conduct an "objective and neutral" inspection of the project to avoid possible criticism over its fairness.

Touching on the inspection, an official said Sunday that the government plans to complete the formation of a task force next month and embark on a full-fledged investigation.

"The investigative committee will be established by the end of May," said the official at the Office for Government Policy Coordination. "To ensure fairness and objectivity, the government will include professors as well as experts recommended by opposition parties in the task force."

The project was completed last year despite mounting criticism.

In January, the Board of Audit and Inspection found that construction work on the four rivers refurbishment project was flawed. (Yonhap News)