In London, investigators know who they're looking for, and there is one glaring profile: Young Middle Eastern or African men with backpacks.
In New York, where searches have been expanded, commuters are picked at random — too random for some.
"I think it's kind of a waste of time, if they're gonna search someone like me," says one young white commuter.
Retired New York police Capt. Stephen Davis — a 20-year veteran — believes the best way to capture a terrorist is through intelligence, not subway searches.
"It's cosmetic," he says. "But at the same time, it may be a deterrent. But in terms of trying to use this as an actual tool to directly combat terrorism and identify people responsible, this isn't the answer."
New York authorities are involved in quite a balancing act — protecting people's rights and their lives. While searches calm the fears of anxious commuters, they can also cause resentment among those who feel targeted unfairly.
"That kind of resentment may very well alienate the communities that you need to have cooperation from in order to get the kind of intelligence as to what kinds of threats are out there," says Robert Cottrol, a professor of law and sociology at the George Washington University Law School.
New York City authorities insist they're not profiling anyone with the random searches.
The New York Civil Liberties Union believes the searches are unconstitutional. But it has only received 25 complaints so far from commuters on its Web site.
Experts warn there can be a danger in looking too closely at any one group.
"If it was very clear that we were profiling and successfully profiling Arab men, they would begin recruiting Europeans or Africans or people of other backgrounds," says Cottrol.
That's what happened in Israel, where women began to replace men as suicide bombers. It's proof that terrorists know how to change their profiles and their tactics.