DANDONG, China — After sunset, when American spy satellites can no longer see as well, the main bridge connecting China to North Korea comes to life.

One by one, trucks laden with Chinese goods rumble across Sino-Korean Friendship Bridge here, long an economic lifeline for North Korea. The covered trucks offer little sign of what might be inside, though Chinese customs data offers clues: heavy machinery, refrigerators, even beer.

The data also suggests something else. The trucks are increasingly coming back empty. And that could present a potential weak point that the Trump administration and others could exploit, if China is willing to go along, as they look for ways to persuade North Korea to give up its nuclear weapons and ballistic missiles.

Even as China sells more goods to North Korea, it is buying less from there, according to trade data and experts on the relationship between the two. North Korea’s finances are hidden by a veil of secrecy. But that worsening trade imbalance appears to be deepening North Korea’s need for credit from China or for cold, hard cash from somewhere else.