The FBI arrested 10 Russian secret agents on June 27 after learning weeks before that one of them, Donald Heathfield of Cambridge, Mass., would soon be traveling abroad with a college-age son and might not return, a U.S. law enforcement official said Monday.

Heathfield's planned departure was in the official's words the big catalyst in deciding to take down a spy network that had been under surveillance by the FBI for more than a decade.

Heathfield's real name is Andrey Bezrukov. The official said Heathfield was to have started late last month. The FBI had reason to believe Heathfield might not be coming back, said the official, who spoke about the matter on condition of anonymity because the official was not authorized by the government to discuss it.

Two White House officials said Friday it became clear in early June that at least two of the Russians were making plans to leave the U.S. The officials did not identify the two, but the law enforcement official said one of them was Heathfield. According to one of the two criminal complaints in the case, another of the Russian agents, Anna Chapman, was planning to leave in mid-July for Moscow.

Preparations took time, once the decision was made to dismantle the Russian network.

The FBI spent weeks preparing a 37-page complaint that a federal magistrate signed June 25, two days before the arrests.

It charged Heathfield, whose real name was his wife and seven other people with two conspiracies — acting as unregistered foreign agents for Russia and engaging in money laundering.

A second complaint, dated June 27, the day of the arrests, charged two people, including Chapman, the daughter of a Russian diplomat. In separate incidents in New York and Washington on June 26, Chapman and the other defendant named in the second complaint were both approached by FBI undercover agents posing as Russians.

Adding to the sense of urgency surrounding the arrests was Chapman's behavior on June 26. She became suspicious when meeting that Saturday with the undercover FBI agent who posed as a Russian consulate employee. The undercover agent asked Chapman to deliver a phony passport to another deep cover Russian agent, but Chapman did not do that.

The court documents show that right after meeting the undercover agent, Chapman bought a one-time-use cell phone under an assumed name.

Then, authorities say, Chapman made a "flurry of calls" to Russia. In one of the intercepted calls, a man advised her she may have been uncovered, should turn in the passport to police and get out of the country. She was arrested the next day at the police station where she tried to turn in the fake passport.

(This version corrects by deleting reference to prior trip overseas.)