A sting operation over the course of months. Federal agents posing as al-Qaeda operatives. The text of the sacred Koran used to send coded messages.

When federal authorities arrested Farooque Ahmed, a 34-year-old Pakistani American, this week for an alleged plot to bomb Metrorail stations in Northern Virginia, Muslim groups in the area struggled with what to say publicly.

Should they condemn the man unequivocally and praise law enforcement? Or should they wait?

As details of the arrest trickled out, many in the Muslim community avoided saying anything to outsiders, but instead quietly voiced concerns to one another about the tactics used.

The ambivalence highlights the complicated and often fraught relationship between law enforcement and Muslim Americans- an alliance some say has suffered especially in the last year with the slew of stinglike operations within their communities.

Increasingly, Muslims think that even as they work with the FBI to combat terrorism, they are being spied on by authorities.

The impact of those suspicions has been profound. Imams say longtime attendees at their mosque have suddenly grown reticent to welcome new strangers and new converts. Some Muslims say they catch themselves watching what they say and to whom, eyeing people in their own community as potential informants looking to lure them into arrest.

"The relationship with law enforcement right now is tense," said Ibrahim Hooper, spokesman for the Council on American-Islamic Relations, a group the FBI stopped working with last year on outreach efforts. "There's a sense of being under siege in many Muslim communities. People just assume there are agents or informants in their mosque now. It's a fact of life."

The FBI, however, emphasized that it does not investigate communities or mosques but individuals. "We are going to go within constitutional parameters where we need to go and talk to people we need to talk to," said spokeswoman Katherine Schweit. "But we are not looking to infiltrate anything other than individuals looking to harm others."

Since last year, more than 60 U.S. citizens have been charged or convicted in terrorism cases, federal officials say.

At Dar al-Hijrah, Imam Johari Abdul-Malik said his mosque works closely with law enforcement. But at the same time, he said, he thinks there's been a chilling effect from the sting operations.

"When someone new walks into our mosque now, people don't talk to them," he said. "White converts have it worst. People just look at them and think, hmm, a little too straitlaced, a little suspicious, probably an agent. They just tell them to go talk to me."