I don't understand the "if not before" part of your comment. Would you please explain what you mean.
FYI, here is an estimate of the number of slaves smuggled into the US illegally between 1808 and 1861 [Link]:
In 1807 the U.S. Congress passed the Act to Prohibit the Importation of Slaves, which ended large-scale importations of slaves into the United States. The law went into effect on January 1, 1808. In the eight years before the Act to Prohibit the Importation of Slaves made the trade illegal, the United States imported about forty thousand new slaves from Africa. From 1808 until the Civil War broke out in 1861, less than a fifth of that number of slaves would be illegally smuggled into the nation.
That totals to about 8,000 slaves in 53 years, or about 150 per year. That is equivalent to maybe one ship per year slipping through. These were illegally imported slaves, typically imported in Northern ships IIRC. Were Southern states themselves behind this illegal importation or was it the act of individuals sneaking ships into backwaters to offload slaves illegally.? I do not know.
In contrast, some Northern states actually passed laws to prevent the return of states to the South, forgetting a great compromise in the Constitution that made the Constitution possible. In addition to this nullification of the Constitution by some Northern states, many individuals in the North opposed the return of fugitive slaves to their owners in the South. On at least one occasion a Northern mob killed the person seeking to return slave(s) to their owner(s).
It was the apparently unconstitutional actions of the Northern states themselves that concerned Southern leaders. Once states started seceding, a number of Northern states repealed or amended their offending laws, but it was too late to save the Union as it was.
I imagine that on occasion free blacks who were not fugitive slaves were captured in the North and taken to the South. However, about ten times as many slaves escaped as were returned. Under the Fugitive Slave Law there were Federal commissioners set up to determine if a captured person was a fugitive slave or not. If he determined to the best of his ability that the person was a fugitive slave, any laws of Northern states regarding the fugitive and his rights no longer applied. The Fugitive Slave Law was ruled constitutional by the Supreme Court.
Secessionist sympathizers often quote a "states rights" myth when discussing the Civil War, which falls flat when looking how long they continually violated the rights of other states.
"Historian Henry Brooks Adams explains that the anti-slavery North took a consistent and principled stand on states' rights against Federal encroachment throughout its history, while the Southern states, whenever they saw an opportunity to expand slavery and the reach of the slave power, often conveniently forgot the principle of states' rights—and fought in favor of Federal centralization:
Between the slave power and states' rights there was no necessary connection. The slave power, when in control, was a centralizing influence, and all the most considerable encroachments on states' rights were its acts. The acquisition and admission of Louisiana; the Embargo; the War of 1812; the annexation of Texas "by joint resolution" [rather than treaty]; the war with Mexico, declared by the mere announcement of President Polk; the Fugitive Slave Law; the Dred Scott decision — all triumphs of the slave power — did far more than either tariffs or internal improvements, which in their origin were also southern measures, to destroy the very memory of states' rights as they existed in 1789. Whenever a question arose of extending or protecting slavery, the slaveholders became friends of centralized power, and used that dangerous weapon with a kind of frenzy. Slavery in fact required centralization in order to maintain and protect itself, but it required to control the centralized machine; it needed despotic principles of government, but it needed them exclusively for its own use. Thus, in truth, states' rights were the protection of the free states, and as a matter of fact, during the domination of the slave power, Massachusetts appealed to this protecting principle as often and almost as loudly as South Carolina.[4]
Sinha[5] and Richards[6] both argue that the states' rights that the Southern states claimed were actually:
States' rights to engage in slavery;
States' rights to suppress the freedom of speech of those opposed to slavery or its expansion, by seizing abolitionist literature from the mail;
States' rights to violate the sovereignty of the non-slave States by sending slave-catchers into their territory to enforce the Fugitive Slave Law of 1850, to seize supposed runaway slaves by force of arms.
States' rights to send armed Border Ruffians into the territories of the United States such as Kansas to engage in massive vote fraud and acts of violence; see Slave Power and Bleeding Kansas;
States' rights to deem portions of their population "beings of an inferior order, and altogether unfit to associate with the white race, either in social or political relations, and so far inferior that they had no rights which the white man was bound to respect", by means of the Dred Scott decision;
States' rights to secede from the United States after an election whose result they disagreed with, the election in 1860 of Abraham Lincoln;
States' rights to seize forts and arsenals of the United States following their purported secession; see Fort Sumter; "Historians, like contemporaries, have long noted that an overwhelming majority of South Carolinians were for secession. But a majority of South Carolinians had nothing to do with secession or the glorification of human bondage. A majority of South Carolinians in 1860 were slaves."
States' rights to have a less democratic form of government; Sinha, in particular, argues this point, illustrating that the state of South Carolina, home of John Calhoun, the ideological godfather of the Slave Power, had a far less democratic order than the several other United States. Although all white male residents were allowed to vote, property restrictions for office holders were higher in South Carolina than in any other state.[3] South Carolina had the only state legislature where slave owners had the majority of seats.[3] It was the only state where the legislature elected the governor, all judges and state electors.[3] The state's chief executive was a figurehead who had no authority to veto legislative law.[3]
States' rights to overturn the ideal expressed in the Declaration of Independence — that "We hold these truths to be self-evident, that all men are created equal, that they are endowed by their Creator with certain unalienable Rights, that among these are Life, Liberty and the pursuit of Happiness".
Sinha and Richards conclude their cases by arguing that the Civil War had nothing to do with "states' rights", democracy, or resistance to arbitrary power. They argue that it was instead the result of the increasing cognitive dissonance in the minds of Northerners and (some) Southern non-slaveowners between the ideals that the United States was founded upon and identified itself as standing for, as expressed in the Declaration of Independence, the Constitution of the United States, and the Bill of Rights, and the reality that the slave-power represented, as what they describe as an anti-democratic, counter-republican, oligarchic, despotic, authoritarian, if not totalitarian, movement for ownership of human beings as the personal chattels of the slaver. As this cognitive dissonance increased, the people of the Northern states, and the Northern states themselves, became increasingly inclined to resist the encroachments of the slave power upon their states' rights and encroachments of the slave power by and upon the Federal Government of the United States. The slave power, having failed to maintain its dominance of the Federal Government through democratic means, sought other means of maintaining its dominance of the Federal Government, by means of military aggression, by right of force and coercion, and thus, the Civil War occurred."
http://en.wikipedia.org/wiki/States'_rights