Is the U.S. naval blockade on Iran working? There appear to be some cracks, with tracking data showing several ships leaving from Iran and transiting the Strait of Hormuz.

The blockade, which was launched Monday, is supposed to prevent Iran from exporting its oil and other goods to pressure it into a deal.

But the reality on the sea is murkier. Kpler, a tracking firm, reported that several ships sailed from Iran through the strait both before and after the 10 a.m. deadline Monday. How they managed to slip through remains unclear. While some ships may have slipped through the cracks, it’s possible that a grace period was extended for a certain amount of time or that some ships received special permission to pass through the blockade.

Among the ships that made it out is the Christianna, a Liberia-flagged cargo ship, which reportedly held no cargo. Following that was Elpis, carrying methanol, which had been sanctioned under its previous name, Chamtang, for its ties to Iranian oil. The Murlikishan, a previously sanctioned oil tanker, also entered the Persian Gulf Tuesday.

But two Chinese ships changed course and turned to Oman before the blockade began. One of them remained in the Gulf of Oman, while the other made another attempt and succeeded in pushing through.

Ships are still moving through the blockade, which was designed to choke off Iran’s economy, raising questions about its effectiveness.