PORTLAND, Ore.  The arrest in a plot to bomb a popular Christmas tree-lighting ceremony here has renewed focus on the crucial but often fragile relationship that many Muslim communities have with federal law enforcement agencies.

Many Muslim leaders nationwide say they are committed to working with the authorities to fight terrorist threats and applauded the work in Portland. But some say cases like the one in Oregon, in which undercover agents said they helped a teenager plan the attack, risk undermining the trust of Muslim communities that federal agents say is essential to doing their jobs.

The failed Portland plot is one of several recent cases, from California to Washington, D.C., in which undercover agents helped suspects pursue terrorist plans. Some Muslims say the government appears to be enabling and even sensationalizing threats that can lead to backlashes against Muslim communities.

On Sunday, a mosque in Corvallis, Ore., was firebombed. It had been attended by the Portland suspect, Mohamed Osman Mohamud, 19, a naturalized American citizen from Somalia.