An investigation of poison gas attacks carried out by Saddam Hussein's regime two decades ago has led Dutch authorities to Maryland, where for more than 18 months they have been quietly gathering evidence about an international businessman's dealings with a now-defunct chemical manufacturer.

Their target is Frans Van Anraat, a Dutch citizen scheduled to stand trial in the Netherlands within weeks on charges of genocide and war crimes. Van Anraat, 63, is accused of supplying the Iraqi regime with thiodiglycol, a key ingredient in the mustard gas used to poison thousands of Iranians and ethnic Kurds in northern Iraq.

Van Anraat's thiodiglycol was manufactured in Baltimore by the chemical company Alcolac Inc., which, U.S. authorities say, effectively supplied both sides during the Iran-Iraq war.

Alcolac pleaded guilty in 1989 to knowingly violating export laws in the case of a shipment of thiodiglycol that ultimately went to Iran.

The recent work of Dutch investigators here suggests that the trial will highlight the Maryland company's role, knowingly or not, in helping Saddam build his arsenal. The company has since been sold and restructured.

"Alcolac turned a blind eye to abundant evidence in its files that this chemical was not going to the final destination that its customers stated in documents filed with customs," said Martin S. Himeles Jr., the former assistant U.S. attorney who prosecuted the case against the company.

The prosecution of Van Anraat is a victory for U.S. Immigration and Customs Enforcement agents and federal prosecutors in Maryland, who indicted Van Anraat in 1989 only to watch him slip away from investigators in Europe. Italian authorities arrested Van Anraat in Milan in 1989, releasing him later after a judge found that the charge against him was a "political crime."

Van Anraat then disappeared, reportedly slipping into Iraq and living as a guest of the regime for more than a decade.

After the government fell, Van Anraat returned to Amsterdam, where he was arrested last year. As early as March 2004, Dutch authorities met with immigration agents and federal prosecutors in Baltimore to share information about the case, ICE spokesman Dean Boyd said.

"Obviously, this is a man who is alleged to have supplied Saddam with chemical weapons, so we certainly wanted to do everything we could to help the Dutch with their case," Boyd said, "and ICE had information from its own long-term investigation dating back to 1984."

Van Anraat has maintained his innocence. His lawyer has not denied that Van Anraat supplied the chemical to Iraq but has said he did not know that it would be put to such a use.

To learn more about Alcolac's role, the Dutch authorities also spent three days interviewing Gary B. Pitts, a lawyer representing Gulf War veterans in a lawsuit against Alcolac, Pitts said.

Pitts said yesterday that the Iraqi government's disclosures to weapons inspectors suggested that Van Anraat was among the regime's largest suppliers of that particular chemical.

Federal prosecutors also aided the Dutch. "The U.S. attorney's office has opened up all of the files that were relevant to the Dutch and their inquiries," said Assistant U.S. Attorney Harvey E. Eisenberg, the district's anti-terrorism coordinator.

In recent days, Dutch authorities, assisted by Eisenberg, have visited the region to question a former CIA political analyst on Iraq as well as former residents of northern Iraq, according to court filings by the Dutch.

The roles of the Iraqis in the case could not be learned.

Saddam's regime used mustard gas and other poisons in assaults on the Kurdish village of Halabja, where an estimated 5,000 people were killed in 1988, and elsewhere.

Prosecutors in the Netherlands accuse Van Anraat of shipping hundreds of tons of chemicals to Iraq, even after the government's chemical attacks on the Iraqi army and on its own Kurdish population were known.

According to wire service reports, Van Anraat's lawyer has disputed those claims.

In a Dutch television interview two years ago, Van Anraat admitted shipping chemicals to Iraq but maintained his innocence. "This was not my main business," he said, according to news accounts. "This is something I did in passing."

Yesterday, Maryland U.S. Attorney Rod J. Rosenstein said it is critical for U.S. law enforcement authorities to assist other countries in such prosecutions.

"And we expect to get similar cooperation from them in return," he said.