An assassin, offered $9,000 to gun down a key witness in a narcotics case last month, lay in wait for 17 hours at the suburban motel where the victim, Scott Piechowicz, worked until the killer found the chance to do the job, federal authorities said in court today.

The dramatic disclosures came after FBI agents arrested Vernon Evans Jr., 33, and Janet Patricia Moore, 21, late Tuesday night on charges of conspiracy and witness tampering, ending one of the most intense manhunts in recent memory here.

The two were ordered held without bond today by U.S. Magistrate Paul M. Rosenberg. Evans, a diminutive man wearing a blue jacket, and Moore, sporting large gold-colored earrings, said little during their appearance before Rosenberg.

U.S. Attorney J. Frederick Motz named Evans as the trigger man in court today, saying a government witness has told investigators that Evans "was the one responsible for the murders" at the motel.

While federal prosecutors do not have authority to charge murder in this kind of a case, they have charged Evans and Moore with conspiracy to violate the civil rights of Piechowicz and hindering his ability to testify by the use of physical force.

Motz told Rosenberg that state charges of murder are being considered against the two, and possibly others, by Baltimore County prosecutors.

The suspects were arrested in connection with the machine-gun slayings April 28 of Piechowicz, 27, manager of the Warren House Motor Hotel in suburban Pikesville, and his sister-in-law, Susan Kennedy, 19, an employe at the motel. Piechowicz had been scheduled to testify at a trial beginning May 2 against Anthony Grandison, an accused drug trafficker.

Piechowicz was prepared to testify that he found drug paraphernalia in a motel room occupied by Grandison. Grandison was convicted in federal court last week of handgun possession and possession of heroin with intent to distribute it.

Though Grandison has not been directly implicated in the case, an FBI affidavit filed in federal court today said that Evans visited Grandison in Baltimore City Jail on April 28, two days before the gangland-style killings, and later told his girlfriend, identified as Charlene Sparrow, that he would receive $9,000 for "pulling it off."

The affidavit, based largely on information provided by Sparrow, who reportedly has been cooperating with the government, said that she rented a room at the Warren House motel late on April 27. Evans joined her some time later, the affidavit said, and throughout the evening, he instructed Sparrow to go to the lobby to see if Piechowicz and his wife, Cheryl, were there. After each trip, Sparrow reported not seeing the couple, according to the affidavit.

The next day, Sparrow and Evans "continued in their residence at the Warren House Motor Hotel," the affidavit said. Sparrow again checked the lobby and reported seeing a couple at the desk, who in fact were Scott Piechowicz and Susan Kennedy, according to the affidavit. (Cheryl Piechowicz subsquently told reporters she was scheduled to work that day but Susan Kennedy substituted for her.)

After Sparrow told Evans of seeing the couple, according to the affidavit, she and Evans left their motel room and later got into a car in the motel parking lot where Sparrow saw Evans handling a small machine gun.

Next, according to the affidavit, Evans got out of the car and walked toward the motel, leaving Sparrow in the car. Moments later, it said, Evans came running back to the car, "carrying the weapon in its carrying case." The couple then fled, the affidavit said.

Piechowicz and Kennedy died in a burst of gunfire from a high-powered machine gun, described in news reports here as an Ingram MAC-10 capable of firing 1,200 rounds a minute. Cheryl Piechowicz, who was also a witness in the Grandison narcotics case, has said she believed the bullets were intended for her and that Kennedy, her substitute that day, was killed by mistake.

Janet Moore, the woman arrested with Evans in the case, was described in the FBI affidavit as a girlfriend of Grandison who helped set up the jailhouse meeting between Evans and Grandison, and drove Evans and Sparrow to the motel.