Defense attorney Barbara Bergman said the others set up Nichols to take the blame for the deaths of 168 people killed in the April 19, 1995, blast at the Alfred P. Murrah Federal Building.
"This is a case about manipulation, betrayal and overreaching," Bergman said. "People who are still unknown assisted Timothy McVeigh."
Nichols, 49, is serving a federal life sentence for involuntary manslaughter and conspiracy in the deaths of eight federal law enforcement officials in the bombing. In this state court trial, he is accused of 161 counts of first-degree murder for the deaths of the other victims plus one victim's fetus.
After the end of defense closing arguments, prosecutors will have an opportunity to present rebuttal before the case goes to the jury, probably on Tuesday afternoon.
If Nichols is convicted, the trial will enter a second phase in which jurors will decide whether he should be sentenced to life in prison or death by lethal injection.
Judge Steven Taylor ruled the death penalty may be sought on all the counts if Nichols is convicted, except one for the death of the fetus.
Defense attorneys had sought to allow jurors to find Nichols guilty of charges less serious than first-degree murder, but Taylor rejected that request.
On Monday, prosecutor Lou Keel said in his closing arguments that suggestions that McVeigh, who was executed in 2001, received substantial help from others does not relieve Nichols of responsibility for his role.
But Bergman reminded jurors of dozens of witnesses who testified they saw McVeigh with others in the weeks before the bombing. Witnesses said the others did not resemble Nichols.
Bergman attacked scientific evidence presented by prosecutors during 29 days of testimony, including the discovery of ammonium nitrate crystals on a piece of plywood that was part of the truck that delivered the bomb.
The bomb was made up of an explosive mixture of ammonium nitrate fertilizer and fuel oil.
Steven Burmeister, an FBI scientist, testified for the prosecution that the crystals were embedded in the plywood under pressure, possibly from an explosion, but former FBI chemist Frederic Whitehurst testified the crystals were only adhering to the surface and that their origin was unknown.
Prosecutors say said Nichols and McVeigh planned the bombing to avenge the government's siege at the Branch Davidian compound near Waco, Texas, which ended with the deaths of 81 people on April 19, 1993 — exactly two years before the bombing.
Keel said McVeigh parked the truck that delivered the bomb on the federal building's east side so the blast would be felt in the offices of the U.S. Bureau of Alcohol, Tobacco and Firearms, which had played a pivotal role at the Waco siege.
Questions about the possible involvement of other accomplices in the 1995 bombing have swirled since McVeigh was put to death after a month's delay because of the belated discovery of some 4,000 pages of documents that had not been turned over to McVeigh's defense during his trial.
In some of the recent revelations: