U.S. security officials have directed the two carriers with nonstop flights from Moscow to the United States -- Delta Air Lines and Aeroflot Russian Airlines -- to screen all passengers and their carry-on and checked luggage for explosives.

The Transportation Security Administration ordered the new security measures in light of evidence that terrorists brought explosives aboard two Russian flights that crashed simultaneously last week, killing 89 people.

All cargo loaded onto Delta and Aeroflot planes must also be screened for explosives.

Neither of the carriers was involved in the two crashes, which occurred on domestic flights operated by Russian carriers. "At this point, we've determined it's prudent to take these additional steps until we have information to assess the ongoing situation," said Amy Von Walter, a TSA spokeswoman.

Some Homeland Security officials and safety experts speculated soon after the crashes that their timing could not have been accidental, even though Russian officials at first denied that there was a terrorist role. The TSA said it waited until this week to require the tighter security measures, after the explosive hexogen, also known as RDX, was found at the crash sites. Russian investigators said they are still investigating whether two women with Chechen surnames, one aboard each plane, might have been terrorists.

The day after the Aug. 24 crashes, U.S. security officials said they had decided not to ask military jets to escort inbound commercial flights from Russia. But later in the day, local residents reported seeing fighter jets following an Aeroflot plane into the Los Angeles area on an unusual path over the ocean. The North American Aerospace Defense Command, the military unit responsible for domestic air defense, decided to follow the flight to Los Angeles because it was conducting exercises in the area, said a senior Homeland Security official who spoke on condition of anonymity because of the sensitivity of military operations. U.S. security and airport officials said they were not aware that any other military jets had since followed commercial jets from Russia.

A NORAD spokesman said he could not comment on military operations.

Delta said it has recommended that passengers on its daily flight to New York's John F. Kennedy International Airport arrive three hours before scheduled departure at Moscow's Sheremetyevo Airport for the additional security procedures, according to spokeswoman Peggy Estes. Representatives of Aeroflot, which has scheduled flights to Washington, New York and Los Angeles, could not be reached for comment.

"We're paying attention to the request of the Americans and we understand the security concerns," said Yevgeniy Khorishko, spokesman for the Russian Embassy.