The Golden Gate Bridge is an American icon and it's been listed as among top targets. But if the bridge has attracted the terrorists for its symbolic value, there are far less famous parts in the Bay Area that more attractive for different reasons.
The terrorist attacks on a Madrid train in 2004 killed 191 people. The London bus attacks in 2005 killed 52, and 700 were injured.
These attacks are grim reminders of just how vulnerable mass transit is.
Brian Jenkins is a counter-terrorism expert who now heads San Jose's Mineta National Transportation Security Institute.
"Terrorists see public service transportation as a killing field," said Jenkins.
Jenkins says mass transit is attractive to terrorists because of relatively easy access. Large numbers of people in a confined space makes for maximum damage -- and the confined spaces can enhance the effects of some explosives.
"For terrorists who are determined to kill, kill in quantity and are willing to killing indiscriminately, public transportation systems are the attracted. They extremely difficult to defend," said Jenkins.
Jenkins says the bay area's BART system has one advantage over others, its relatively newer than rail systems in the big east coast cities, making it easier to update.
BART has invested $47 million in counter-terrorism measures since 911. It has state-of the art surveillance systems and dogs trained to sniff out explosives and weapons.
BART has received $19 million in federal security grants, but says it still needs to $250 million to reach its counter-terror goals.
"We keep talking about $250 million but every time we get a little bit in, it's still $250 million because costs go up," said BART board member Lynette Sweet.
The Golden Gate Bridge District says it's made improvements in surveillance, motion detectors and lighting. It's received $6 million in homeland security funding.
"We're prepared for as many possible scenarios as we can be prepared for. We go through risk analysis, contingency planning," said Mary Currie from the Golden Gate Bridge District.
And at SFO, the airport's security operations are constantly being updated.
"The important thing for the public to understand is that we are always looking to improve the system. We are always looking to raise that bar there, and there is no finish line in this process because the people who want to breach these systems, they are not taking a step back, they are moving forward so we need to be one step ahead of them," said Mike McCarron from SFO.
Jenkins said it's not realistic to think about trying to control our surface mass transit systems like we do our airports. He just wrote a book that takes a look at whether terrorists might be acquiring nuclear weapons. He'll be talking about that and transportation security this month at San Francisco's Commonwealth Club.