That informant also proposed undertaking an undercover operation against the Mexican airstrip, sources said.
The memo was revealed Monday by attorneys for LaRouche, who is on trial in Boston on charges of credit card fraud.
The attorneys contend that North directed the FBI to gather information against LaRouche and infiltrate his organization. They say they intend to prove that the charges against LaRouche were concocted as part of a government plot to ruin LaRouche because he refused to support President Reagan's programs to aid the Nicaraguan contras.
The attorneys, who have requested thousands of government documents in preparation of their case, argued Tuesday and yesterday for release of additional classified FBI files.
The Secord memo, retrieved from the safe that North used as a National Security Council aide, had been in the hands of the independent counsel investigating the Iran-contra affair.
"Our source reports that terrorists plan to use airfield near Texas border. Strip is at intersection of Marfa vor 280 radial and Hudspeth vor 168 radial," Secord wrote to North, using aviation coordinates to locate the airstrip.
"Lewis has met with FBI and agency reps and is apparently meeting again today," he continued. "Our man here claims that Lewis has collected info against LaRouche. Let's see how polygraph goes."
According to several sources, Lewis is Fred J. Lewis, a former Army Delta Force commando who had served with an aide to Secord, John Cupp.
LaRouche attorneys contend that "our man" is Oliver Revell, an FBI assistant director involved in counterintelligence.
Secord, however, said in a telephone interview that "our man" referred to Cupp. Cupp, who has been interviewed by the congressional Iran-contra committees, provided personnel, logistics and other support to the secret contra supply network operated by Secord under North's direction.
"Cupp figured I knew the best way for them (Lewis and another informant) to get a hearing," Secord said. "It had nothing to do with us," he said, referring to himself and North.
Revell previously has testified that he had an arms-length relationship with North and that he handled all of North's requests in conformity with FBI guidelines.
The Federal Aviation Administration's El Paso Sectional Chart locates the private airstrip described by Secord near Rancho del Pino, about 18 miles south of the U.S.-Mexican border.
In April 1986, an airstrip employee told Mexican drug lord Pablo Acosta that Colombians, Nicaraguans and Arabs ran a thriving trade in arms and drugs
from the remote mountain facility, according to Capt. Clayton McKinney of the Midland County Sheriff's Office. Acosta, an informant who passed that information on to U.S. officials, was slain by Mexican federal agents in April in a shoot-out backed by the FBI.
About the time of the U.S. bombing of Tripoli on April 19, 1986, three Libyans were caught near El Paso seeking to enter the United States illegally, according to Midland County Sheriff Gary Painter. Again, Acosta provided the tipoff, Painter said in a telephone interview.
Lewis, a part-time sheriff's deputy, and a second deputy, Gary Howard, proposed to Cupp that they set up a $24,000-a-month undercover operation against the Mexican airstrip. They proposed to give information to Secord through Cupp.
Cupp seemed interested, well-placed sources said, particularly if Sandinista, contra or Libyan operatives were involved in the arms and narcotics traffic.
Earlier, in October 1984, Howard had sought to interest the FBI and CIA in infiltrating the LaRouche organization. Both agencies at first seemed interested. The idea was deferred until after the 1984 election, then dropped. Secord's memo suggests it came up again in 1986.
Lewis and Howard offered to take the polygraph exam referred to in Secord's May 1986 memo, several sources said, but the FBI could not provide an examiner. No action was ever taken against the enterprise.