After listening to 21 days of testimony and arguments, a U.S. District Court jury yesterday began deliberating the case against three anti-Castro Cubans on trial in connection with the 1976 bombing assassination of former Chilean ambassador Orlando Letelier.

The seven-woman, five-man jury, which heard 33 witnesses and watched as more than 125 exhibits were introduced into evidence, started its deliberations at 3:30 p.m., but had not reached a verdict by early last night. The jury resumes deliberations at 9 a.m. today.

To the end, the biggest contention in the case was over the testimony of the government's key witness, Michael V. Townley, an American-born Chilean secret police agent who testified that he assassinated Letelier on orders from his superiors in the secret police, formerly known as DINA. Ronni K. Moffitt, a colleague of Letelier's at the Institute for Policy Studies, also was killed in the Sept. 21, 1976, explosion on Embassy Row, and her husband was injured.

In their closing arguments on Monday, defense lawyers characterized Townley, among a variety of disparaging descriptions, as a "baldfaced liar," an "animal" and "a man who talks about eliminating people as if they were bugs."

But yesterday, in his rebuttal, Assistant U.S. Attorney E. Lawrence Barcella Jr. tried to turn the defense arguments against the three defendants -- Guillermo Novo Sampol and Alvin Ross Diaz, who are charged with the two murders, and Novo's brother Ignacio, who is charged with lying to a grand jury investigating the Letelier killing and failing to tell authorities about the crime.

"If Michael Vernon Townley is a coldblooded assassin, and he is," Barcella said, "then what kind of people are Guillermo Novo and Alvin Ross?

"They are mercenaries without conscience," the prosecutor told the jury. "These people are in the same league as Michael Vernon Townley. They are assassins."

Barcella took note the defense's effort to point out "mistakes and inconsistencies" in the testimony given by the prosecution's 25 witnesses. But Barcella said that such mistakes and inconsistencies "don't rise to the level of lies."

The prosecutor said that the defense had "glossed over the facts, ignored documentary evidence and tried to put everyone on the defense except the defendants."

Defense lawyers in the case suggested to the jury that Townley may have killed Letelier on orders of the Central Intelligence Agency. Townley was conditionally approved by the CIA for use in an "operational capacity" in the early 1970s, but the CIA has said that it had no contact with Townley since 1973.

Late yesterday afternoon, the jury sent Judge Barrington D. Parker a note asking to see nine charts produced by the government for the case. In addition, the jury asked for all testimony and government plea-bargain agreements with Townley and two other convicts who testified for the prosecution, extortionist Sherman Kaminsky and self-described con artist Ricardo Canete.