Officials disbanded a police academy class and dispatched recruits to stake out bridges in an effort to crack the case of Atlanta's dead or missing young blacks, the jury in the Wayne B. Williams trial was told today.

The move was prompted by police beliefs that the killer or killers had started throwing victims into the water to wash off the fiber clues on the bodies--clues that had been well publicized in the local media.

Williams, 23, a freelance TV cameraman and self-styled talent scout, is accused of murdering Nathaniel Cater, 27, and Jimmy Ray Payne, 21, two of the victims whose bodies were found in the river. Until February, when reports of the fiber clue had been published, no bodies were found in the river, according to testimony Wednesday.

A task force commander testified today that police were so desperate for a break they staked out the bridges with raw recruits like Freddie Jacobs.

Just before 3 a.m. on May 22, the prosecution contends, Jacobs saw a car driven by Williams moving slowly from a bridge seconds after something made a loud splash in the water below.

Jacobs, on the stand today, testified that Rob Campbell, a fellow recruit, asked him: " 'Freddie, I just heard a big loud splash down here, is there a car on the bridge?' "

"I saw a car there," Jacobs said. It was a battered, white Chevrolet station wagon driven by Williams.

"It was extremely close" to the bridge, the young stakeout officer told a packed courtroom. "It appeared to come from a parked position. I would say the speed of the vehicle was in the neighborhood of three to four miles per hour, awfully slow."

Until Jacobs spotted Williams, police had no weapon, no crime scene, no clues except tiny green fibers found on some victims. All they had was bodies, and in many cases, including Cater's, the same cause of death--strangulation. Payne died from asphyxiation, a medical examiner has testified.

Minutes after Jacobs spotted his car, Williams was pulled over near the bridge and grilled for two hours. Two days later, he became a key suspect when Cater's body was found 1.2 miles downstream. Payne's body was found near the same spot a month earlier.

The prosecution's strategy appears to be a recreation of the random and bizarre events as they appeared to police investigating the case that lead to the bridge encounter and Williams' subsequent arrest. The case against the defendant would come together like "pieces of the puzzle," the district attorney promised.

Indeed, the case against Williams apears to be coming together in a random fashion, as if Fulton County District Attorney Lewis Slaton were putting on a mystery play for the jury, allowing the evidence to come out the same way investigators got it.

To help the jurors decipher the evidence, the prosecution supplied them with a science lesson from a civil engineer with the Army Corps of Engineers.

Benjamin Kittle, the engineer, testified about Chattahoochee River currents, specific gravity and how a body would bounce along the bottom before gases in decomposition made it rise to the surface.

Kittle scraped chalk across a blackboard to explain the water-displacement principles of Archimedes and defined the specific gravity as the "algebraic sum of gravitational pull and the velocity of water."

Under cross-examination, he said it would be possible for Payne's body to have entered the river in several places, but a police detective armed with dozens of color photographs said the river was virtually inaccessible in that area, save from the Jackson Parkway Bridge where Williams was stopped.