The

Blackstone Group

slashed by $2 a share its four-month-old offer to buy

Kroll-O'Gara

(KROG)

, Kroll said Tuesday.

The companies offered no explanation for why the offer was being reduced to $16 a share.

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Kroll, the corporate detective firm known for ferreting out improper uses of insider information, made the announcement after trading was temporarily halted Tuesday afternoon. But Kroll shares closed down only 7/16, or 3%, at 13 7/8 after trading as low as 10 5/8 prior to the announcement.

The company's stock, however, had fallen more than 2 points on Monday, hitting a low of 14 before closing at 14 5/16, down 1 15/16 for the day. It was the first time the shares had closed below 15 1/2 since the deal was announced on Nov. 15.

At the time,

Blackstone Capital Partners III Merchant Banking Fund L.P.

, a division of New York-based investment bank Blackstone Group, offered $18 a share for the bulk of the company's stock. The $475 million deal would have left Jules B. Kroll, chairman of Kroll-O'Gara, owning approximately 10% and remaining chairman.

A Kroll spokesman did not immediately return calls for comment, and Blackstone could not immediately be reached.