NAIROBI, May 28 (Xinhua) -- Kenyan police have arrested about 20 suspects from the National Youth Service (NYS) over a 90 million U.S. dollar graft scandal.

Youth Affairs Principal Secretary Lillian Mbogo-Omollo presented herself to the police on Monday as investigators intensify probe into the scandal at the NYS, the country's premier vocational training institute for under-privileged youth, involving some of its flagship projects.

The arrests were ordered on Sunday by Director of Public Prosecutions (DPP) Noordin Haji after he independently reviewed all the ten inquiries files related to the ongoing investigations at NYS and directed that prosecution commence immediately against all 48 named suspects.

"A team of experienced prosecutors led by the DPP is currently undertaking a final review of the files," Haji said.

"The DPP assures the public that appropriate directions based on analysis of evidence and the law shall be issued without undue delay," he added.

Among those arrested are NYS director-general Richard Ndubai and top finance and procurement officials. They were held at various police stations ahead of their prosecution this week.

The suspects are likely to spend another night there ahead of their prosecution on Tuesday.

"We spent the night looking for them. They will also spend theirs in custody ahead of their prosecution, probably on Tuesday," said an official with knowledge of the operation.

The arrests of senior government officials signal looming arrests and prosecutions of more than 40 other suspects, including NYS managers, suppliers and treasury officials.

Reports indicate that on Friday, Haji and DCI director George Kinoti met to review statements recorded by the individuals suspected to have been involved in the latest NYS scandal.

Investigations were launched after it emerged that an unknown amount of money could have been lost at the NYS through fictitious payments.

President Uhuru Kenyatta has ordered speedy investigations that have seen Omollo and Ndubai step aside. All NYS procurement, finance and accounting personnel who are under investigation have been sent on compulsory leave.

A multi-agency team pursuing allegations of fraud in paying tenders is embarking on the second phase of their probe.

Investigators want to establish if there are any case of collusion between suppliers and public officers that may have led to theft of money in fictitious claims involving billions of shillings (tens of millions of dollars).