Enrique, the bald Avis clerk at Tampico International Airport, was taking forever with our rental car. He kept looking from our licenses to Dominic, the photographer, and me, presumably befuddled as to why two Americans would be coming to Tamaulipas state.

Resigned to giving us the keys, he finally said, “Just don’t stop. Don’t fill the tank inside the city. Don’t buy anything from Oxxo,” the ubiquitous Mexican convenience store. “There are people watching, and they will know Americans are in town.”

He looked gravely worried.

“Things are happening, often,” he said. “You need to take precautions.”

We encountered that kind of worry and fear repeatedly during our stay in Tampico, a wealthy port city now almost paralyzed by fighting between the Gulf drug cartel and the Zetas, as well as within the Gulf cartel. One of the strange things about the place is the contrast between its veneer of modernity and the attitudes of its residents. The city’s main drags are populated by familiar chains: Wal-Mart, Starbucks, IHOP, TGI Fridays, KFC, Home Depot, Church’s Chicken. Interspersed are bars and restaurants that have been strafed by bullets and burned by cartel arsonists.

The local news media, which are warned not to report on cartel activity, observe an almost total information blackout. So residents share details on social networks and via e-mail groups, including the list of 15 bars, restaurants and nightclubs that have been burned or shot up, or closed down voluntarily, in the past two months.

The recent upsurge in trouble, and the residents’ belief that their government is complicit in cartel activity, made people feel helpless about their options and frightened to talk about their problems. Few people we spoke to wanted to have their names published. We met with medical students who made sure they were alone with us in a tiny office before they would tell us about their worries.

“The walls have ears,” one of them said.

One of the few people who spoke on the record, Lubin Jimenez Horak, a well-known radio host, was wary enough about doing so that he recorded himself responding to our questions.

The soldiers and police patrolling the streets were on a hair trigger. When Dominic photographed a passing truck full of Mexican soldiers, they tailed us for a few blocks and then pulled us over, shouting that unauthorized photographs were illegal. Dominic mentioned that he had recently taken many photographs of Mexican soldiers in another state, Michoacan.

“This is not Michoacan,” a soldier said angrily. “This is Tamaulipas.”

Over the past year, residents across the western half of Michoacan have taken up arms to fight the Knights Templar cartel themselves, after years of ineffective military operations. No such citizen militia movement has emerged in Tamaulipas, despite similar acts of extortion and kidnapping by the cartels. Carlos Heredia, a professor in Mexico City who was born in Tampico and has worked as an adviser to the governor of Michoacan, cited several reasons for the differing citizen response.

The lowland Tierra Caliente area of Michoacan where the rebellion started is more cohesive, with farming families spread across the area, than Tamaulipas’s cities along the U.S. border and the Gulf Coast, he said.

“Tamaulipas does not operate as a unified single state,” he said. “You have municipalities that are isolated, and not necessarily linked to other regions. There is a highway, but it’s not the same families, people, nothing. It’s a state that has developed in a fragmented way.”

The cartel bosses in Tamaulipas, Heredia added, do not necessarily come from the cities and “plazas” they control, and don’t really care about the residents. In Michoacan, many of the Knights Templar leaders were from the villages they controlled — and built lavish mansions there among their impoverished neighbors, who knew them well.

There have been glimmers of citizen activity in Tamaulipas. Thousands of residents wearing white have marched in recent peace rallies in Tampico and Ciudad Victoria. From his computer, Eduardo Cantu, who sells alarm systems and bottled water and does Internet trading, has emerged as a protest leader, organizing support online. His group’s Facebook page, launched last month, now has more than 12,000 members.

“I don’t think we need weapons to defend ourselves,” he said. “We have to invest our time and talent and money in order to help the city.”