Six suspects have been released from "investigative detention" in a €162m ($181m) fraud probe involving troubled Croatian shipbuilder Uljanik Group.

The county court in Rijeka made the order on Monday, the Hina news agency said.

Court spokesman Zoran Srsen said that there were no longer grounds for detention because all the witnesses had been interviewed during the investigation.

The six are former Uljanik Group management board chair Gianni Rossanda; its former management board members Marinko Brgic and Veljko Grbac; Maksimilijan Percan, ex-chairman of subsidiary yard 3 Maj; former Uljanik Plovidba director Dragutin Pavletic and former Uljanik Shipyard director Silvan Kranjc.

Eight other people were reported to be suspects.

The investigation relates to the terms of contracts for four newbuildings signed in 2010.

In March, two executives of Croatian shipowner Uljanik Plovidba quit following their arrests in relation to the probe.

The tanker and bulker owner said Pavletic and Anton Brajkovic, deputy president of the supervisory board, tendered their resignations.

No mention of the shipyard investigation was made in the statement by Uljanik Plovidba.

Pavletic was effectively the company's managing director.

Last summer, the shipowner hit back at what it called "make-believe" news reports seeking to link it to, or blame it for, the ongoing crisis at Uljanik Group.

The company pointed out it is completely independent from its former parent group.

Pavletic said he had resigned from Uljanik Group's supervisory board on 23 July 2018, to avoid the two companies being perceived as linked.

He joined the board in June 2017.

The Croatian government is trying to secure Chinese investment for the yard group, which has been hit by worker strikes and order cancellations.