The telephone at Terry G. Blackstock's home rang repeatedly one sweltering Sunday last July. The caller, identified as Bob Anderson, said he was in town for just a few days and emphatically needed to see Blackstock. Although Blackstock was out for the day, Anderson left no call-back number.

Less than 24 hours later, Blackstock -- deeply in debt from a number of unsuccessful business deals -- was found dead of gunshout wounds on an isolated road in southeastern Fairfax County where he had gone to look over some real estate with the same Bob Anderson.

Yesterday, Fairfax County police said they believe that Anderson was a professional killer hired to murder Blackstock, 28, the troubled vice president of an Alexandria home-remodeling firm. The suspect, descriibed as "a real mystery man" by a police spokesman, is the subject of an extensive investigation that now involves police in Baltimore, Tampa, Philadelphia, and Pittsburgh.

"It is our inclination to believe that 'Bob Anderson' is an assumed name, and that the primary suspect may have been recruited from one of those cities. said Fairfax police spokesman Warren Carmichael yesterday.

Police refused to elaborate, except to say their information was based on more than 100 interviews conducted since the slaying.

The disclosure is the latest development in the case of the young businessman who, friends say, was living in fear for days before his death last July 14.

"I think his life had been threatened," said a close friend yesterday. "Someone had seen him [Blackstock] carrying a gun and when they asked him why, he said someone had threatened him about getting money."

The friend, who asked to remain anonymous, said a gun was found in the trunk of Blackstock's car after he was slain.

Blackstock founded the five-year-old Blackstock Building Co., which he operated for a time from his home on Belmont Boulevard in Fairfax, according to his father, James. Two years ago, the business fell on hard times and Blackstock returned to the Alexandria Redevelopment Co., where he had worked previously.

He remained heavily in dept, however, and his home was often deluged with telephone calls from creditors, according to friends and police. He was described as in "a more than cheerful mood" on the day of his death. But friends yesterday portrayed him as a worried man, unusually tense and irritable during the week before his murder.

During several telephone calls on July 13, Anderson said he had seen a group of town houses in Arlington built by Blackstock and wanted Blackstock to build a similar house for him, according to a friend. The friend said the calls upset Blackstock because he was unable to determine how Anderson had linked him to the Arlington development. But Blackstock later dismissed his suspicions.

At 2 p.m. on July 14, Carmichael said, Anderson appeared at the Alexandria company's offices: a white male, aged 26-30, with thick, dusty blond hair and clad in jeans and an unusually bulky, blue and white sweater, despite the 90-degree heat outside.

The two men talked in Blackstock's office for 15 minutes before they left in Anderson's silver-blue sedan, ostensibly to look at Anderson's "property," Carmichael said. At 3 p.m., Blackstock called his office to check for telephone messages, giving no indication of any problems. Carmichael said police believe Blackstock may have made the call from his home, seven miles from the scene of the murder, and that Anderson was probably still with the victim at the time.

Blackstock was found dead 30 minutes later by William Tuerke Barrett, who had been visiting relatives at a sprawling, 65-acre estate on Gunston Road near where Blackstock was found. Barrett then drove past a man walking along Gunston Road who fit Anderson's description. At the time, Barrett said, "It was strange because there's really no place to walk to from there. We gave each other a long hard look."

Carmichael said police in Tampa, Baltimore, Pittsburgh and Philadelphia have been supplied with composite drawings of the suspect.