Probably this or a similar action. After the passage of the Fugitive Slave Act, there was increasing reluctance by the Northern states to help enforce this Federal Law, since no trial was held in the state where the purported slave had fled to. They saw it as a violation of states rights. This varied in extremity from time to time and place to place but there was harassment of slave catchers and at least this pair of deaths.
Forcible Resistance. When legislative appeals and litigation campaigns broke down, some Northerners resorted to direct action to
rescue alleged slaves. When Georgia slave catchers arrived in Boston a
few weeks after the passage of the Fugitive Slave Act to bring back
the runaways William and Ellen Craft, abolitionists hid the couple
while a vigilance committee harassed the agents into leaving the town.
A runaway from Virginia named Shadrach, seized in February 1851 in a Boston coffeehouse by
slave catchers to whom he was serving coffee, was rescued from a
federal courthouse by a group of African Americans who overpowered the
marshals guarding the fugitive. The most violent clash took place in
the Quaker community of Christiana, Pennsylvania, in September 1851,
where a Maryland slaveowner was killed and his son seriously wounded
in a gunfight with a group of African Americans resisting an attempt
to seize three former slaves. Shortly afterward a group of black and
white abolitionists broke into a police station in Syracuse, New York,
rescued a captured runaway known as Jerry, and helped him cross Lake
Ontario into Canada.