After twin bombings killed three people and wounded over 170 others at Monday's Boston Marathon, many Americans are asking how safe the United States is from terrorists' mayhem.

Two bombs went off Monday afternoon at the annual Boston Marathon, ripping through flesh and severing limbs in a powerful explosion that left an 8-year-old boy and a Chinese female student among the dead.

While former President George W. Bush and President Barack Obama have devoted enormous resources to fighting terrorism and thwarting attacks, it only takes one bomb to create chaos, and experts said other threats are increasing.

A report penned earlier this year by West Point's Combating Terrorism Center found a "dramatic rise" in attacks and plots by domestic far-right extremists. The report defined such militants as those who "espouse strong convictions" that the federal government is "corrupt and tyrannical."

Another concern is that the Department of Homeland Security grants to help U.S. states buy equipment and draw up response plans for terror attacks have dwindled from a high of 2 billion U.S. dollars in 2003 to 294 million U.S. dollars last year.

The sequester - a spate of spending cuts that kicked in on March 1 - will also cause funds to drop another 5 percent, according to Stateline, a news service from the Pew Charitable Trusts.

Critics have blasted the Department of Homeland Security for its allocation of state grants, as it disproportionately awarded grants to smaller states with fewer homeland security risks.

Of the top five states that received funding, only Washington, D.C. is likely to be hit by a terrorist strike. New York, which saw more than 2,000 casualties after the Sept.11, 2001 attack on the World Trade Center, is 10th on that list, Stateline reported.

Michelle Majewski, editor-in-chief of the Journal of Homeland Security Education and professor at Marian University in Wisconsin, told Xinhua that the United States is also vulnerable in the realm of cyber security.

"If you look at technology, the Internet, those are all vulnerable, we've got a long way to go to shore all of that up," she said, but added that the business sector, as well as local, state and federal government are working on the issue.

Moreover, the U.S. has made major strides in securing the country over the last decade since 9/11, she said.

"That said, we can't prevent every single incident from happening. It's just not practical. We do the best job that we can do through intelligence and making communities aware and getting people to pay attention to what's going on," she said. "But there are incidents like this (Boston bombings) that do happen."

Indeed, experts noted that the U.S. has not seen a terror attack against civilians since the 9/11 attacks in New York City and Washington more than a decade ago.

"In a complex world with a number of disenfranchised groups and largely open borders... (the attack in Boston) should not in and of itself be surprising," wrote global intelligence company Stratfor on its website Tuesday.

"Rather, the length of time that Americans have not faced a major foreign-inspired or domestic terrorist event on their shores is perhaps more remarkable," it argued.

A number of attacks on the U.S. have been foiled over the last decade - many of them by ordinary bystanders. Passengers aboard a U.S. jetliner headed for Miami tackled shoe bomber Richard Reid in 2001 when he tried to set off an explosive device in his footwear.

In another case, bystanders foiled a Christmas Day, 2009 terror plot when underwear bomber Umar Farouk Abdulmutallab tried to explode a bomb hidden in his undergarments aboard a Detroit-bound flight.