U.S. Headed for 'Heightened Alert' Stage Exclusive: Major events on the horizon prompt a surge in anti-terror efforts.

July 28, 2008 -- Government officials have been quietly stepping up counterterror efforts out of a growing concern that al Qaeda or similar organizations might try to capitalize on the spate of extremely high-profile events in the coming months, sources tell ABC News.

Security experts point to next month's Olympics as evidence that high-profile events attract threats of terrorism, like the one issued this past weekend by a Chinese Muslim minority group that warned of its intent to attack the Games.

Anti-terror officials in the U.S. cite this summer and fall's lineup of two major political parties' conventions, November's general election and months of transition into a new presidential administration as cause for heightened awareness and action.

This is what the Department of Homeland Security is quietly declaring a Period of Heightened Alert, or POHA, a time frame when terrorists may have more incentive to attack.

According to drafts of government memos described to ABC News, the period would run roughly from this August through July 2009.

During this time, homeland security analysts will be asked to redouble efforts to study terrorism leads. And a number of agencies will be asked to review emergency response plans to a variety of attacks, from improvised explosive devices (IEDs) to biological weapons.

Officials also are being asked to make sure they are prepared for all contingencies during the transition from the Bush administration to that of the next president.

In a recent interview, FBI director Robert Mueller told ABC News of his concerns for homeland security.

"When you have a series of events like this which are very public, where you have a number of people that are congregated together, we take additional precautions," he said.

"That means identifying, focusing on the intelligence that's available and scrutinizing it to pieces and running it to ground, to putting in place the precautions to assure the particular events go according to plan and free from terrorist attacks," he said.

At the moment, the nation's public threat level will remain at yellow, or "elevated," but not orange, or "high."

The reasons: There are no specifics indicating an attack on the U.S. is imminent, and U.S. officials do not want to be accused of trying to inject themselves into the presidential campaign.

"That's a balancing act," said Jerry Hauer, former Homeland Security official and ABC News consultant. "They really have to focus on these events and this critical time we're going through as a nation, but they have to be very careful about the public message to not make it look political or like they're fearmongering."

Government officials point to the Sept. 11 attacks, which happened just nine months into a new administration, and the Madrid train bombings, which were carried out just three days before Spain's 2004 general election.

They say history suggests a need to take potential threats seriously -- especially in the very near future.