An accused Spanish arms dealer, Monzer al-Kassar, will ask a federal judge tomorrow to release him from American custody, arguing that federal agents violated international law by setting up the sting operation on Spanish soil that led to his capture last year.

Mr. Kassar, whom prosecutors have described as one of the world’s most prolific arms dealers, now faces trial in America on charges that he planned to send more than 7,000 rifles and missiles to the coca-funded terrorists in Colombia who are known as the Revolutionary Armed Forces of Colombia, or FARC. Mr. Kassar and two co-defendants are charged with plotting to kill Americans based on the government’s allegation that the missiles were intended for use against American military helicopters in operation in Colombia.

Mr. Kassar’s trial in Manhattan, should there be one, could also highlight evidence the government said it has uncovered of ties between Mr. Kassar and Palestinian Arab terrorists, as well as with Saddam Hussein’s regime.

In court papers, prosecutors claim that an associate of Mr. Kassar’s told the Drug Enforcement Administration that the accused arms dealer had sold 12 million euros worth of night vision goggles to Saddam’s son Uday in the months prior to the American-led invasion of Iraq, government court papers say. And prosecutors say the same associate, who is not identified in court papers, also claimed that Mr. Kassar arranged for a plane to fly $1 billion out of Iraq.

A confidential report by a DEA agent also states that Mr. Kassar maintained contact with a Lebanese terrorist, Samir Kuntar, even as Kuntar was imprisoned in Israel for the 1979 murder of three Israelis, including a 4-year-old girl. Kuntar, whom Israel released last month in exchange for the bodies of two kidnapped Israeli soldiers as part of a deal with Hezbollah, apparently had access to a telephone smuggled into the Israeli prison, according to the DEA report, which is included as a court exhibit. The report is based on information from a post-arrest interview with one of Mr. Kassar’s co-defendants, Tareq al-Ghazi.

The government’s case appears to have suffered a setback recently with the murder of one of its primary witnesses. The witness is one of three confidential informants for the DEA who posed as an arms purchaser for FARC, according to government court papers. The informant had been present for at least one meeting in early 2007 at Mr. Kassar’s estate in Marbella, Spain.

A spokeswoman for the U.S. attorney’s office in Manhattan, Yusill Scribner, declined to provide any details surrounding the murder. Government court papers filed last week say only that the informant, identified as Luis, was “recently murdered at a location outside the United States.”

At a court hearing tomorrow in U.S. District Court in Manhattan, lawyers for Mr. Kassar and two co-defendants are scheduled to argue that the charges are without precedent because they involve “citizens of other nations having no connection to the U.S.” and a sting operation “conducted wholly in other countries,” according to court papers filed by Mr. Ghazi, the co-defendant, whose legal papers Mr. Kassar has joined.

The defense lawyers are trying to get the indictment dismissed on the grounds that the “crimes here were invented by the U.S. Government.”

Citing the international nature of the sting operation, the defense motion argues that the case represents “a precarious and monumental step in the history of American law enforcement.”

One aspect of the case that could prove salient at a trial is how little it seems that Messrs. Kassar and Ghazi knew about FARC, which has been waging a guerrilla war against the Colombian government for decades, until they were approached by the informants. Defense lawyers say the DEA’s sources “gratuitously regaled the defendants with tales and motives of the FARC, such tales being met generally with polite rejoinders or more commonly, utter indifference,” the defendant’s legal papers say.

At one point during conversations at the Marbella estate, Mr. Kassar asked one of the confidential informants whether FARC was “against or with” America, according to a government brief that characterizes the conversations.

In a court affidavit, Mr. Kassar states that the DEA informants represented themselves to him as representatives of the Nicaraguan government, not FARC, who were seeking to purchase arms.

Tomorrow’s hearing, before Judge Jed Rakoff of U.S. District Court in Manhattan, could focus on the defense’s assertion that American agents violated an international treaty by keeping the sting operation secret from Spanish authorities.

Mr. Kassar was tried and acquitted in Spain more than a decade ago at a trial of charges of piracy and providing arms to the Palestinian Arab terrorists who hijacked the Achille Lauro cruise ship. Those allegations could come up again at trial, as Mr. Ghazi, the co-defendant, is alleged in court papers to have helped the mastermind of the hijacking, Abu Abbas, escape afterward.