Bernard C. Welch, serving a sentence of 143 years to life for the 1980 murder of prominent Washington cardiologist Michael Halberstam, escaped with another convict from a barless downtown federal prison here Tuesday night. The two men escaped by breaking through a concrete wall and descending six floors to the street using a 75-foot electric extension cord.

Welch, 45, and Hugh T. Colomb, 31, smashed a slit-like, sealed window only three inches wide, enlarged the opening to about six inches, wiggled through and climbed down. It was the first major escape in the 10 years that the 26-story, wedge-shaped Metropolitan Correctional Center has been in operation in the South Loop.

The escape was discovered when the sole guard on duty on the maximum-security sixth floor realized at 9:48 p.m. that two prisoners were missing.

Chicago police said they were not notified of the escape for several hours. Wanted bulletins have been circulated for the two, who are considered "extremely dangerous."

Colomb was serving 48 years on voluntary manslaughter, armed robbery and other charges.

Welch and Colomb had escaped from prison before, but Warden O.C. Jenkins said he was never informed of this.

Jenkins said the two were brought here separately from maximum-security Marion Prison in southern Illinois several months ago because they had offered law enforcement authorities information about possible escape plans at other federal prisons.

At a news conference, Jenkins said the two had been talking to U.S. prosecutors about alleged breakout attempts. The Justice Department refused to confirm or deny this.

Welch was convicted in D.C. Superior Court of first-degree murder for slaying Halberstam, who surprised Welch burglarizing the physician's Northwest home on Dec. 5, 1980. Welch shot Halberstam and fled on foot.

The physician, brother of Pulitzer Prize-winning journalist David Halberstam, headed for the hospital by car. Accompanied by his wife, he spotted Welch in the street and ran him down. Halberstam, a respected physician with a national reputation, then lost consciousness and the auto hit a tree. He died at Sibley Memorial Hospital.

At Welch's trial four years ago, defense attorney Sol Z. Rosen declared that Welch "is of the opinion that he can break out of any prison and does not intend to serve his sentence."

Welch escaped from a New York prison in 1974 and lived a comfortable life as a burglar preying on the homes of well-to-do Washingtonians until his apprehension in the Halberstam murder. Police discovered several million dollars' worth of valuables in Welch's $235,000 Great Falls, Va., home; in a $100,000 Duluth, Minn., summer home; and in his silver Mercedes-Benz.

Colomb once escaped from the Rutland, Vt., Correctional Center and was recaptured. He was convicted of stabbing a fellow inmate to death in 1979 while serving time at Marion Prison for a 1977 Los Angeles bank robbery.

Federal prison records show that Welch had been transferred within the federal prison system five times since his conviction of first-degree murder in the Halberstam slaying.

Beyond saying that the Chicago guard had not been injured, Jenkins had few details about how the escape occurred. He said he did not know what tools were used to chip through the wall. He said he had no knowledge of possible accomplices.

Chicago police speculated that the two used barbells or other weights from the prison gym to smash their way to freedom. The prison holds 353 inmates, but usually contains about 100 more. It is considered an administrative facility, and can handle every security category, from minimum-security, work-release prisoners to those requiring maximum control.

The center was designed by Harry Weese and Associates, the Chicago architecture firm that designed the Washington Metro stations.

Jenkins said the maximum security floor has 14 one-man walled cells and a communal area. Each cell has a solid door with a small window. Prisoners are allowed to close but not lock the door.

Marshals described Welch as white, 6 feet 1, 180 pounds, with brown hair and blue eyes. Colomb is white, 5 feet 10, 158 pounds, with brown eyes and brown hair. Both were dressed in blue denim prison clothing. Persons with information of their whereabouts were asked by the U.S. Marshals Service to call, toll-free, 1-800-336-0102.

The $10.2 million prison, at 71 W. Van Buren St., stands a few blocks from the Sears Tower and other Chicago financial district landmarks.

The facility has no wall, fence, guard posts or other security barriers around it. The embrasure windows, set deep within reinforced concrete walls, measure three inches wide by 10 feet high. Until Welch and Colomb broke out, bars were thought to be unnecessary.

In Washington, U.S. Attorney Joseph E. diGenova said he had been told by federal prosecutors here that Welch and Colomb were in Chicago as part of a federal investigation. He said Colomb was a member of the white-supremacist Aryan Brotherhood.

Joseph Crabtree, a spokesman for the federal Bureau of Prisons, said this is the second time that Welch has been in the Metropolitan Correctional Center in Chicago.

Crabtree said Welch initially was sent to the federal penitentiary in Marion on May 28, 1981, and stayed there almost two years. He was transferred to the Chicago prison on April 7, 1983, and remained there until Nov. 5 of that year when he was transferred to the federal prison in Sandstone, Minn. On March 7, 1984, he was taken to the federal penitentiary in Atlanta. Welch was brought to Chicago for a second time on March 1 of this year, Crabtree said.