Mexican officials are investigating an accident earlier this month at a fabrication yard owned by a joint venture of Fluor Corp. and Empresas ICA, S.A.B. de C.V. that caused workers working in a man-basket to fall 130 ft to the ground, killing five instantly. Two others later died from their injuries.

The firms also confirmed that one worker is in serious condition after the June 11 accident at the Matarredonda facility in Veracruz state.

ICA-Fluor’s work there is part of its $95-million contract, awarded in 2012, to build the 11,832-ton Ayatsil-C offshore oil platform that will be installed in the Gulf of Mexico for Mexican oil giant Pemex.

According to Fluor, six of the dead workers are employed by ICA-Fluor and one by ESE ASA, a Mexican industrial-crane rental and transportation firm.

In a statement, Mexico City-based ICA said the man-basket being held by a 600-ton Manitowoc crane fell with the workers in it. The crew was removing platform scaffolding when the accident occurred, but the number of workers in the basket was not disclosed.

ICA said the accident’s cause is “unknown,” but one emergency official speculated that the cause was related to mechanical failure or a severed cable, said Reuters.

A report by Bloomberg said the accident was Fluor-ICA’s deadliest since the joint venture formed in 1993.

“We will allocate the necessary resources to determine as soon as possible the cause of the accident and implement any necessary preventive measures,” said the ICA statement.

Fluor also is working with investigators, but the firms’ spokespersons did not update the status of work at the fabrication yard. ICA, which says it is Mexico’s largest infrastructure firm, owns 51% of the venture.

Fluor said that the platform, reaching a depth of 132 meters, will be the first structure of its size to be installed by Pemex. The project had been set for completion by the end of 2013, but its new end date is not clear.