BONN, West Germany (AP) _ A prosecutor said today he has opened a criminal investigation involving officials of a company that contracted to deliver to Iran chemicals that could be used to make poison gas.

The Rheineisen Chemical Products company of Duesseldorf said Wednesday that it canceled its contract to deliver the thionyl chloride from a company in India. Government officials said they were investigating whether the deal violated export laws.

Rolf Chanteaux, the chief state prosecutor in Duesseldorf, where the company is based, said he opened a criminal probe Wednesday afternoon that will focus on Rheineisen officials.

Chanteaux told The Associated Press the company is suspected of violating West German foreign trade laws by allegedly failing to get a special permit for the deal. He gave no further details.

West German officials began looking into the case after U.S. officials tipped off investigators the thionyl chloride shipment could be used to make mustard gas, authorities said.

The incident raised questions about how tough West Germany's new export control efforts are following last year's scandal over German links to a suspected Libyan chemical weapons plant.

''It may be comforting that West German officials are now investigating,'' ZDF television network said in a newscast late Wednesday. ''Not so comforting is the thought that we would again be totally in the dark without the tip from the United States.''

The Duesseldorf company insisted West German customs officials initially assured the firm there was nothing wrong with shipping the 257 tons of thionyl chloride to Iran.

''A customs employee confirmed on May 22 that there were no objections to the shipment,'' company attorney Georg Greeven told reporters Wednesday night.

Greeven claimed West German customs authorities didn't alert the company until Wednesday that under new export control laws adopted April 7, the firm might need a permit for its involvement in the Iran transaction.

He said Rheineisen was able ''at the last moment'' Wednesday to keep the first delivery of the chemicals from being loaded aboard a ship in India. He did not identify the ship or the port or give other details.

Greeven said the chemicals were made by a company in India called Transpec. He did not give the location of the firm.

He also did not indicate whether Rheineisen knew the chemical could be used to make mustard gas.

Iran used mustard gas in its eight-year war with Iraq, which ended with a cease-fire in August, and there have been unconfirmed reports Tehran is trying to add to its chemical weapons stockpile.

Rheineisen is run by an Iranian family and was contracted to arrange the shipment by Iran's state-owned industrial organization Vezarate Defa, Greeven said. He said Iran approached the Duesseldorf company about arranging delivery of the chemical in January.

Under an agreement reached June 1, Rheineisen was to receive $360,000 from Iran, Greeven said. He did not reveal any details on the India company's involvement in the transaction.

U.S. authorities tipped the West German government to Rheineisen's involvement in the deal last week, Bonn officials say.

American officials also alerted West Germany late last year about alleged links between German companies and a suspected chemical weapons plant in Rabta, Libya, about 40 miles outside Tripoli.

The West German government at first insisted the United States had no convincing evidence for its allegations, but later launched its own investigation.

West German authorities have since arrested the head of the Imhausen-Chemie company in connection with the Libyan deal.