Five months after Frederick County businessman Robert Lee Miller Jr. was shot to death in a Rockville motel room, Montgomery County police have no suspects in his death, but are focusing their attention on the flamboyant contractor's business dealings.

Miller, the president of the contracting firm Interstate Bridge Co., was shot in the back of the head in a room at a Ramada Inn in Rockville last May, as his chauffeur waited outside in Miller's silver Lincoln Continenal.

At the time of his death Miller was attempting to buy control of the company he headed, according to his family and other sources.

At the same time Miller had been subpoenaed to testify before a federal grand jury in Baltimore that was probing allegations of labor-management racketeering. In the course of that probe, prosecutors have looked into the activities of a Washington area firm, Excavation Construction Inc. That firm isowned by the same holding company that held a 50 percent interest in Miller's company.

The owner of stock in both companies is a Nevada holding company, ICE, Inc. ICE owns 50 percent of Interstate Bridge and 100 percent of Excavation Construction. ICE or John W. Lyon and Larry A. Campbell who are partners in the firm, also have or have had an interest in several other area construction firms.

Miller owned 37 percent of the stock of Interstate Bridge. Mason Combs. who helped Miller found the firm, owns 13 percent, according to sources and court records.

Both police and Miller's family believe that Miller was attempting to buy out ICE. Miller's son said that he had been told by the attorney representing his father's estate that his father was attempting to buy $300,000 worth of additional stock in Interstate Bridge. Other sources said the amount would have been adequate to buy ICE's 50 percent of the stock.

"I really think it was something to do with the business," Miller's son Kenneth said of the murder.

Miller's wife, Margaret Miller, from whom he was estranged at the time of his murder, said she was also aware that Miller was trying to buy more stock in the business. "I think that's where some of the answers lie," she said.

No official of Interstate Bridge or ICE could be reached for comment on negotiations over the stock ownership of Interstate Bridge, nor did Lyon or Campbell or an attorney listed as president of ICE reply to written inquiries about the negotiations. An attorney who frequently represents Lyon and Campbell replied that he had "no knowledge of any of the matters stated in your questions."

Kenneth Miller said early in the investigation that he believed the murder was business-related because of its location: Miller and other constuction industry officials frequently met in hotels to settle disputes or sign agreements.

The slain man, who lived in an extravagant manner, dressed in custom-tailored cowboy suits and owned several racehorses, was attempting to buy additional stock in the corporation at a time when the business had apparently suffered losses and when he faced substantial personal debt. Miller left behind an insolvent estate but probably could have borrowed money sufficient for the stock purchase, according to a source.

It "appears . . . that the estate of Robert L. Miller Jr. is insolvent, there being more debts than assets to pay said debts," said an attorney representing the estate in a petition to the Frederick County Orphans Court. Miller lived in Frederick County and operated his business there.

In that petition, attorney David S. Weinberg asked for permission to sell a house in Oxford, Md., two lots and the contents of the house to help pay some of those debts.

The house and furnishings were part of a bequest by Miller to his secretary. Frances Diane Wiles, who frequently accompanied Miller to job sites dressed in western wear that matched Miller's.

Miller also left her "any furniture or furnishings which I may own and which may be located in her home near New Market, Md." and "a ring with a yellow diamond setting and a gold bracelet with diamonds, both of which are in a safe-deposit box."

The house and lots in Oxford, which were mortgaged for $245,000, were sold by Miller estate for $115,000. Racehorses that Miller owned at the time of his death also have been sold to help pay his debts.

The will and the nearly $93,000 in claimes filed against it so far reflect much of Miller's style in life - including a taste for expensive jewelry antiques and cars.

To his daughter Deborah, Miller left a blue Corvette; to his son's former wife he left an Oldsmobile, and to his wife he left a red Corvette. Miller left his collection of guns, antique decoys and pictures, "my personal jewelry and any other automobiles" to his son. The rest of his estate went to this family.

However, Kenneth Miller said he expected that the beneficiaries named in the will would end up with little of what was intended because of the debts pending against the estate.

One of the claims against the estate was filed by Colonial Jewelers, asking for $8,977.25 which the firm says Miller owned for past purchases. Included with the claim are receipts that bear notations of purchases such as a sapphire and diamond ring for $1,900; sapphire and diamond earrings for $3,300, and a 14-karat gold necklace for $900.

In addition to the will, Miller left behind insurance, including a $2 million corporate insurance policy on Miller's life of which Interstate Bridge is the beneficiary.

Miller's wife and son said that Miller had indicated that Mrs. Miller was to receive income through the $2 million policy in the event of Miller's death. However, both Mrs. Miller and Kenneth Miller said that they understand that the income may not be paid.

"The company lawyers have told her lawyer that she's not going to get anything," said Kenneth Miller. According to one source, at issue is a raise Miller gave himself sometime before his death. Miller was said to have made provisions for his salary to continue after his death but may have increased his salary without bothering with procedures required by the company's bylaws, according to sources close to the investigation.

The $2 million policy on Miller's life was one of six such policies that covered key officials of the firm. Such policies are a common business practice and usually cover officials who would be difficult to replace immediately.

Mrs. Miller said that she had received some income from small life insurance policies that her husband had at the time of his death although not from the largest policy.

Miller was shot once in the back of the head with a large caliber weapon on May 9, within two hours after he arrived at the Ramada Inn in his chauffeur-driven car. Miller arrived about 6:30 p.m. and checked into a room he had reserved earlier that day. Sometime after he arrived, Miller apparently returned to his car for his gun. Police found $1,900 in cash and a loaded handgun on Miller's body.

The motel manager, who said he was concerned about the odd circumstances of Miller's stay the motel, used a passkey to enter his room and found the body about 8:30 p.m.