In the carnage in Mumbai, the terrorists entered the city from the water, landing at a jetty at the Gateway to India, a popular tourist destination.

Could such a thing happen here? Would terrorist commandos be able to dock at Penn's Landing and attack targets in Philadelphia?

"I'm absolutely convinced it could be done in Philadelphia," said Kenneth Gabriel, a University of Maryland professor who studies port security. "Not by common criminals. But smart, determined people with plenty of resources can find the weak points in a security system."

Coast Guard Capt. Dave Scott, the commander for Delaware Bay, said such an attack can't be ruled out, but you won't find large ships wandering around the Delaware unnoticed.

"Mumbai is not Philadelphia," said Scott, who visited the Indian city when he was stationed in Asia. "It's a city of millions where comings and goings from the harbor are probably not as well-tracked as they are here."

The Mumbai terrorists arrived with weapons and explosives aboard light, fast outboard-motor boats, launched from a larger ship anchored in the harbor.

Any large vessel trying to reach Philadelphia would need a local pilot to navigate up the Delaware, Scott said. To get a pilot, the ship has to give the Coast Guard four days' notice and disclose its flag of registry, its last five ports of call, the nationalities of its crew and the contents of its cargo, he said.

Scott said that the Coast Guard runs that data past U.S. intelligence agencies, and a vessel will get more scrutiny if those agencies raise a flag or if an arriving ship has made recent stops at ports with lax security.

"Sometimes if they're coming from West Africa, where they don't have the security resources we do, we'll board and inspect them, and perhaps ride with them all the way up to the port," Scott said.

Vessels weighing less than 300 tons - a limit high enough to include some deep-sea fishing boats and luxury yachts - can travel up and down the Delaware as they please.

The Coast Guard can't monitor every potential landing spot on the river around the clock, Scott said, but it does patrol the river, as do other law-enforcement marine units.

"We can't stop everything," Scott said, "but with good communication, planning and intelligent risk management, we can make [an assault] from the maritime side more difficult."

"My sense would be if someone wanted to create a mass-casualty event, it would probably be a lot easier to do from the land side."

Gabriel, director of advanced concepts at the University of Maryland's Department of Electrical and Computer Engineering, said that federal authorities have made important strides in port security, but that more should be done.

Gabriel and his team are developing a sophisticated system to electronically tag shipping containers and cargo holds as they're sealed for their voyages and send alerts to authorities if they're tampered with.

"We need to invest in technology, so we're not depending on people to try and figure out what's in thousands and thousands of shipping containers," Gabriel said.

He said port workers should have better background checks and face sophisticated identification screening which incorporates biometric data, such as fingerprints, retinal patterns or facial characteristics.