Americans who’ve grown accustomed to rigorous security procedures since the 9/11 attacks may have to endure new measures following the Boston Marathon bombings, but experts on Tuesday warned that no amount of extra precautions can guarantee absolute safety from a determined terrorist.

“There is no way that people who run in marathons or people who go to baseball stadiums can be assured that they will be protected from IEDs (improvised explosive devices) 100 percent of the time,” said Dennis Pluchinsky, a former senior State Department terrorism analyst. “Terrorists will always find some holes, some gap (in security) to take advantage of.”

Experts noted that improved counterterrorism efforts overseas, security measures and intelligence sharing between federal, state and local authorities and with other countries have succeeded in thwarting major and minor attacks on Americans at home and abroad.

“Literally dozens of plots have been disrupted and prevented since 9/11,” said Christian Beckner, deputy director of George Washington University’s Homeland Security Policy Institute. “For the most part, we’ve been extremely successful in preventing plots and making it much more difficult for al-Qaida … to conduct attacks inside the United States.”

Brian Jenkins, a terrorism expert at the RAND Corp., a California-based policy institute, pointed to what he said was “an extraordinary improvement since 9/11 in intelligence internationally and domestically.

At the international level, the unanimity of focus and degree of cooperation among the intelligence services and law enforcement organizations worldwide is unprecedented.”

At least 37 of the more than 40 suspected al Qaida-related plots since Sept. 11, 2001, have been thwarted “as a consequence of bits of information from different foreign intelligence sources being assembled and passed along,” Jenkins said. “And it ends up with a plot being thwarted in Europe or the United States. That is a remarkable change.”

The last attack linked to al-Qaida took place on Nov. 6, 2009. U.S. Army Maj. Nidal Malik Hasan, a psychiatrist who was exchanging messages with a cleric of al-Qaida’s branch in Yemen, is awaiting trial on charges of killing 13 people and wounding more than 30 others in a shooting rampage at a medical facility at Fort Hood, Texas.

Some U.S. officials and independent experts expressed doubts that al-Qaida was directly involved in the Boston bombings. The terrorist network’s attacks traditionally have been more devastating and have been followed by claims of responsibility, they said.

“It will surprise me if this is not domestic terrorism,” said Michael Greenberger, a former senior Justice Department official. “It would also surprise me if this is al-Qaida. But it can’t be taken off the table.”

Experts cautioned that absolute security can’t be assured at major public events.

“These are soft targets and they’re very, very difficult to defend against,” said Jack Tomarchio, a former principal deputy for intelligence and analysis for the Department of Homeland Security. “I think what happened yesterday is going to be somewhat of a game changer. In the last couple of years, the idea of homeland security has become kind of blase.”

Pluchinsky, the former State Department terrorism analyst, said it is normal for authorities around the country and the world to increase security after a terrorist attack out of a concern that it could be part of a larger plot.

But that has seldom been the case, and taking additional precautions actually rewards whoever pulled off the attack, he said.

“The terrorist gets a measure of success and victory,” he explained. “I’m sure there will be increased security, and maybe at baseball games and basketball games they will take a second look. But I think they’ve already had pretty good security, and I’m not sure there is very much extra they can do.”