Explosives and other bombing devices were shipped into the United States aboard Chilean passenger airplanes on at least four occasions before and after the assassination of former Chilean ambassador Orlando Letelier here in 1976, a congressional subcommittee was told yesterday.

In addition, an agent of DINA, the Chilean secret police, who was not a pilot was allowed to land a loaded Chilean government-owned aircraft in Santiago, the subcommittee was told.

The details of the alleged role played by the airline Lan-Chile in the Letelier assassination plot came out at a hearing of a House transportation subcommittee that is investigating the lack of action against the airline by the two agencies responsible for air safety and air traffic regulation in the United States.

Under insistent prodding by subcommittee chairman John Burton (D-Calif.), representatives of the Federal Aviation Administration said the agency will oepn an investigation into some of the areas brought up at the hearing to see if air safety regulations were violated.

Burton said the allegations reflected "a pattern on Lan-Chile of carrying explosive devices for terrorist activities" and that an earlier FAA probe -- which consisted only of a review of FBI files on the case -- was inadequate.

"In my judgment, you dropped the ball. You didn't really have an investigation," Burton said.

The Civil Aeronautics Board, meanwhile, said it learned of the allegations of Lan-Chile involvement only earlier this week, and began an investigation immediately. The CAB regulates routes and fares for airlines, and could recommend revocation of Lan-Chile's permit to land in the U.S.

One Lan-Chile official testified yesterday that he had transported packages for a man he believed was a Chilean DINA agent aboard Lan-Chile flights, but that he never knew of any explosives being shipped on the airline.

Another Chilean airline official was allowed to testify before the subcommittee behind closed doors. But he also denied any awareness of shipments of explosives or of the agent's being allowed to fly a Chilean passenger plane, it was learned.

Assistant U.S. Attorney E. Lawrence Barcella Jr. told the subcommittee the background of the Letelier assassination investigation, in which it is alleged that the leader of the Chilean secret police ordered Letelier's death in Washington.

A DINA operative, Michael Vernon Townley, was sent to the United States where he recruited people from the militant Cuban Nationalist Movement and supplied them with explosives to carry out the plot, according to trial testimony in the case.

FBI agent Robert W. Scherrer testified yesterday that Townley had told the FBI of four instances in which he shipped plastic explosives or blasting caps on the airline, including one occasion when he carried them on the plane in the passenger compartment.

In addition, Scherrer testified, Townley was allowed to use a Lan-Chile lounge to meet other DINA agents while on his mission to kill Letelier, and used his influence with Lan-Chile to get tickets upgraded for those spies on their return to Chile. He also had Lan-Chile employes rent a car on his behalf, and a Lan-Chile pilot helped him get a ticket under an assumed name for his return to Chile, Scherrer said.

Although there is no indication that Lan-chile was aware of the purpose of Townley's mission here, "they knew something was not correct" because of his use of aliases, Scherrer said. He said the airline should "very definitely" have been concerned about the safety of its passenges because of the shipments of explosives.

FAA security director Richard Lally defended his agency's earlier decision not to carry out a full investigation of the allegations against Lan-Chile, saying there was no indication the airline was aware of the purposes of the assassination plot or the shipments of explosives.

He said airport security systems "aren't infallible" and that the FAA had since been told by the FBI that Townley could have slipped by the security devices in Santiago.

Letelier and a co-worker, Ronni K. Moffitt, were killed on Sept. 21, 1976, when a bomb exploded under the car Moffitt was driving on Sheridan Circle NW near the Chilean Embassy here.