A Montgomery County grand jury investigating county police monitoring of gambling at a private club in Rockville has been extended to look into why the police abruptly stopped investigating the club in 1981, according to the county prosecutor.

The grand jury heard testimony yesterday from members of the Progress Club who, according to State's Attorney Andrew L. Sonner, agreed to testify as part of a plea bargain in a separate court action in March.

Sonner said the current jury probe concerns whether some county police officers acted criminally in allowing gambling at the club, and whether officers assured club members that no arrests would be made despite a years-long police investigation of the club's gambling.

Police Chief Bernard D. Crooke said yesterday that such allegations are unfounded.

The Progress Club went on trial last year on illegal gambling charges, brought after Rockville police officers arrested 21 men, mostly elderly and affluent businessmen, on illegal gaming charges in a raid on the club on June 5.

The high-profile trial ended in a hung jury; when it later appeared that a retrial was imminent, the club sought a plea arrangement and agreed, among other things, to shift its focus from card-playing to other activities.

The grand jury has been meeting since last fall in separate action. The jury began looking at whether top county police officers, including Crooke, ignored gambling at the Rockville club in exchange for contributions totaling $5,500 to two police-affiliated charities in 1983.

Not to continue the grand jury investigation into the other aspects as well "would send the wrong message to the police and to the community," Sonner said. "It would say the prosecutor's office would not investigate the police department when there is a reason to suspect illegal activity -- that there is somehow an old boy network of police and prosecutors, that we will look the other way. And we won't."

In October, the grand jury subpoenaed and received all county police files on the club. The files are expansive until March 1981, when they dwindled to "almost nothing," according to Sonner.

"If we the prosecutor's office had a case that had been developed over the course of years and we dropped it abruptly, I would expect to be investigated," Sonner said.

But Crooke said yesterday that the investigation was active until the files were turned over to Sonner for use by the Rockville police five months before the June raid.

"I'm not aware of any abrupt cessation," Crooke said. "I know I'll work on something, then put it aside when something hotter comes along. The Progress Club wasn't the most horrendous activity going on in Montgomery County in the area of illegal gambling. I'm sure there were weeks or months when nothing was done on the Progress Club. But there was an active investigation."

Crooke said he would welcome the end of the grand jury investigation, adding that talk about possible police corruption has eroded his morale and that of his men.

"It's a long overdue issue as far as the grand jury's concerned. I want this thing terminated as soon as possible to clear the air," Crooke said. "I'm sick of it -- it's about time they took the cloud off the police department."

A county police department investigation of the Progress Club was begun in 1978, before Crooke was appointed chief in May 1979 and before the club moved to Rockville in June 1979. The club moved from the District, where police also had investigated the club's activities.

Crooke said his department's investigation of the club's gambling activities, lacking an inside informant, never yielded enough evidence to obtain a search warrant necessary to raid club quarters at Congressional Plaza Shopping Center on Rockville Pike.

"Nobody ever gave any directions to stop the investigation. In the last conversation I had with police investigators , I said, 'If you can get them, get them,' " Crooke said.

The grand jury, which meets once a month, has not subpoenaed Crooke, but it heard testimony last fall from at least six of the police department's ranking officers.