The special police task force investigating the murders of 28 young blacks here will close its doors for good, Public Safety Commissioner Lee P. Brown said today, two days after Wayne B. Williams was convicted in the killings of two of the victims on the task force's list.

Brown said authorities have evidence tying Williams to 21 other victims, 20 of whom were on the list. Evidence from the murder of the 21st victim was introduced at Williams' trial, as was evidence from nine other murders investigated by the task force with which Williams was not charged.

Brown met today with state, county and federal law enforcement officials and local prosecutors who reviewed the cases and closed the files on them.

"With Wayne Williams' conviction, we've reviewed all the cases and cleared 23," Brown told a news conference.

"Do you believe Wayne Williams committed 23 murders?" Brown was asked.

"Yes, we do," said Brown, speaking at task force headquarters on West Peachtree Street, where the walls were once hung with composite sketches of killers drawn from psychics' visions.

Almost 200,000 bits of information were fed into computer terminals in the downtown headquarters, a converted car dealership. The phones once rang off the hook with tipsters reporting sightings of "the snatcher."

The walls are bare now, save for an area map and a motto--"In God We Trust, All Others We Polygraph." The phones ceased jangling--and the string of similar killings stopped--soon after Williams was stopped for questioning last May 22 when he drove across a bridge over the Chattahoochee River.

Two days later, the body of Nathaniel Cater, 27, washed up downstream, near the spot where the body of Jimmy Ray Payne, 21, was fished from the river a month earlier. Williams was convicted of those murders by a jury of eight blacks and four whites who deliberated for 12 hours over a two-day period.

Those murders were investigated by the special task force, set up in the summer of 1980, and they became part of a string of slayings of young blacks that terrorized the city for a 22-month period beginning in July, 1979.

The task force list eventually contained 28 names. The death of John Harold Porter, potentially a 29th victim, was investigated but his name was never added to the list. Prosecutors introduced evidence in his murder in Williams' trial, however.

Now officials say they can link 11 additional victims from the list to Williams through fibers and other circumstantial evidence. But local prosecutors today ruled out seeking further indictments against Williams.

Fulton County District Attorney Lewis Slaton said he doubted the evidence was sufficient to win additional convictions.

Brown said the files on the remaining cases were being returned to county police for further investigation. Those include the only two female victims, Angel Lanier, 12, and Latonya Wilson, 8. Still open, too, is the case of 10-year-old Darron Glass, whose body has never been found.

Brown said there was no evidence to show Williams had an accomplice. He said there had been no similar killings of young blacks since Williams became a suspect, and he denied reports to the contrary by one television network.

Several mothers of victims have voiced anger at the closing of the task force, and many citizens have expressed doubts of Williams' guilt. Brown admonished them to "look at the trial and the verdict of the jury," and trust officials' "greater access to information" surrounding the killings.