The former daughter-in-law of Frederick County construction executive Robert Lee Miller Jr., whose murder in a Rockville motel 13 months ago remains unsolved, was arrested and bundled into a car by U.S. agents outside a Frederick restaurant Wednesday night.

While onlookers outside the Red Barn restaurant called city police in the belief that 21-year-old Robin Miller had been abducted, two federal agents and an assistant U.S. attorney drove her to Baltimore where she was questioned for a few hours then released after the charges were dropped.

The bizarre sequence of events began when Miller agreed to meet assistant U.S. Attorney Robert Rohrbaugh at the Red Barn at about 6 p.m. Wednesday.

During the course of the conversation, the prosecutor and the two federal agents accompanying him decided that Miller was withholding information from them.

They immediately arrested her on charges of obstructing a criminal investigation and failing to report a felony, according to Edward D. Hegarty, the agent in charge of the FBI's Baltimore office.

Miller was driven to Baltimore, where Rohrbaugh, prosecutor's investigator Pete Twardowicz and an unnamed FBI agent, continued questioning there, Hegarty said. As a a result of that conversation, the charges against her were dropped, Hegarty said.

It could not be determined what information the federal officials were seeking to get from Miller, who was divorced within the last year from Kenneth Miller, a son of the slain contractor and race horse owner.

The murder victim, Robert Lee Miller Jr., was killed two months after he was subpoenaed to testify before a federal grand jury in Baltimore. The jury was looking into allegations of kickbacks and racketeering among unions and construction firms in the Washington and Baltimore areas.

Miller, a flamboyant dresser who was called "Cowboy Bob," and who bet freely on the 30 horses he owned, was president of the Interstate Bridge Company of Maryland Inc., a construction firm based in southern Frederick County.

Miller was found shot in the head in the room of a Rockville motel where he had gone for a business meeting.

Robin Miller, who was separated from her husband Kenneth shortly after her father-in-law's murder on May 10, worker for four years in the insurance division of the Interstate Bridge firm, according to members of her family.

They said she was laid off four weeks ago because there was not enough work for her.

A Montgomery County detective said last night that he did not believe Robin Miller was being questioned about the death of her former father-in-law. The police officer speculated that she probably was being interviewed about the continuing federal grand jury investigation of construction firms and unions.

After Miller was swept away, two customers of the restaurant telephoned the Maryland State Police and reported the license tag of the car. The State Police gave that information to the Frederick City Police, who fed the number into the statewide license tag computer, only to learn that no such number was on file.

Major Richard J. Ashton, the Frederick police chief, said it was not too unusual to be unable to trace a license tag. The chief was upset by the failure to the FBI to notify his 75-member department that it was conducting an investigation and making an arrest in his jurisdiction.

"If our people had gone by there [when Miller was resisting getting in the car], it could have been very embarrassing" if Frederick police officers had attempted to arrest the federal agents, the chief said.

The arrest occurred about 6:15 p.m. Wednesday. Later that night, Frederick police went to Miller's home, a semidetached row house in a workingclass neighborhood five blocks from the Red Barn, to inform her mother, Betty Pearl, of the apparent abduction.

While she was talking to the Frederick police, Mrs. Pearl said, the FBI called from Baltimore and told her they had arrested her daughter and had her in custody there.

Robin Miller had been scheduled to meet with federal officials in Baltimore yesterday, but after she broke that appointment, a man believed to be investigator Twardowicz, returned to the Pearl home and was questioning Miller when a reporter arrived.

Miller refused to answer any of the reporter's questions. CAPTION: Picture, ROBERT LEE MILLER JR. . . . ex-daughter-in-law quizzed in death