For 25 years, Nirwan T. Thapar attended a Rotary club meeting every week, regardless of where in the world he happened to be.

This morning, members of his local Rotary club will gather for their weekly breakfast in Lanham, and their project will be gathering a reward for information about the unknown gunman who killed Thapar, a popular veterinarian, and his wife last week.

Thapar, 61, and his wife, Shashi Thapar, 52, were fatally shot Wednesday afternoon inside their veterinary clinic in the 5600 block of Annapolis Road in Bladensburg. Prince George's County police have not made any arrests and have released few details, although they said robbery may have been a motive.

Investigators have circulated a sketch of a possible suspect, a man that witnesses told police was near the animal hospital about the time the couple was shot.

Police said it is unclear whether the Thapars knew their attacker and have declined to say whether anything was stolen from the clinic, known as Bladensburg Animal Hospital. How the killer got inside is also a mystery; patrons said that the door to the clinic had a buzzer lock and that visitors could not enter unless somebody let them in.

"We are still investigating and are looking into a couple of leads," said Sgt. Gary Cunningham, a police spokesman. Neither of the Thapars' adult daughters could be reached for comment.

Meanwhile, neighbors and community leaders said they are mourning the Thapars, who were active in service organizations in Bladensburg and Lanham. Nirwan Thapar had operated the hospital, which treated small animals, since purchasing the practice from another veterinarian in March 1993. His wife sometimes worked as a receptionist when things were busy, friends said.

"He was just a stalwart in the community," said Joe Rodriguez, co-owner of the Americana Grocery in Bladensburg, who served with Thapar on the board of the Bladensburg Local Development Corp., a merchants group. "He is going to be missed tremendously. He was like a rock. He was always there."

On Saturday, more than 500 people attended a funeral service for the Thapars in Arlington. Although details of the reward have not been finalized, the Prince George's police Crime Solvers program has agreed to contribute $1,000, said Cpl. Diane Richardson, a police spokeswoman. Police asked anyone with information about the slayings to call 301-735-1111 or 800-673-2777.

The Thapars were natives of India. He was from the Punjab region of northern India; she was from New Delhi. Friends from the Rotary Club said the couple moved to Maryland in 1976 when Nirwan Thapar got a job with the U.S. Department of Agriculture.

Thapar later operated a veterinary clinic in Anne Arundel County for several years before taking over Bladensburg Animal Hospital from a retiring veterinarian.

Larry Maske, who owns a sheet-metal manufacturing company nearby, said Thapar was dedicated to his patients.

"I wouldn't take my animals to anyone else," Maske said of his Labrador retriever, Raven, and his two cats, Muffins and Bill Wilson. "He was very gentle. He knew his trade very well."

Nirwan Thapar joined the Rotary, an international service organization, in 1973, while serving a veterinary internship in San Diego. Thereafter, for more than a quarter-century, he never allowed a week to go by without attending a club meeting somewhere, fellow Rotarians said.

For more than 20 years, he belonged to the Rotary Club of Severna Park and held a variety of leadership posts. About a year ago, when he and his wife moved from Anne Arundel to Prince George's, he transferred his membership to the fledgling Lanham Rotary, which had formed a few months earlier.

In Lanham, he quickly assumed a number of responsibilities. He served as membership director and was in charge of an international service program that benefited an eye bank in India.

"To have a member of his status was a great boost," said Larry Hudson, a past president of the Lanham Rotary. "To lose him is a great loss for our club. It's absolutely a terrible tragedy."

CAPTION: Shashi Thapar and Nirwan Thapar, fatally shot last week at their veterinary clinic in Bladensburg, are mourned by neighbors and community leaders.