A theft during an open house at an Upper West Side apartment last weekend is causing real estate brokers to worry the thieves could strike again.

Two women posing as apartment hunters — one of whom is believed to have been a man dressed in women’s clothing — were caught red-handed on Sunday attempting to steal belongings, according to the apartment’s listing broker, Douglas Heddings of Prudential Douglas Elliman. He asked that the apartment’s address not be published.

The suspects escaped, and Mr. Hedding then reached out to colleagues and provided descriptions of the duo. He said he received a flurry of responses from agents who said they had also been victimized by the duo. In the past three weeks, the suspects have struck at least five times, mostly on the Upper West Side, Mr. Hedding said.

A real estate agent who works for Mr. Heddings caught the two suspects — a woman with blonde hair and one with brunette, who is thought to be a man — in the act. The building provided Mr. Heddings with security footage that appears to show the brunette adjusting a wig.

The thieves escaped with some jewelry that held sentimental value for the apartment owner, and left behind some telling clues, Mr. Hedding, who first reported the incident on his Web log True Gotham, said.

“They left a box of jewelry, cufflinks, and prescription narcotics belonging to several different people,” he said.

Police also found other property belonging to the apartment owner stashed in odd places throughout the home. After being caught, the thieves apparently unloaded the goods before fleeing.