TrendPulse|Waco, OKC bombing and Columbine shooting: How the April tragedies are (and aren't) related

The TrendPulseanniversaries of three American tragedies occur this weekend.

Friday marks the 31st year since the end of the Waco siege and the 29th since the Oklahoma City bombing. Saturday will be the 25th anniversary of the Columbine High School shooting.

The events of those days, which collectively took 257 lives, have served as landmarks in American history demonstrating the capabilities of far-right terrorists and the unofficial beginning of the age of school shootings.

The siege at Waco was cited by the man primarily behind the Oklahoma City Bombing, Timothy McVeigh, who believed that what happened to the Branch Davidians at Waco was "dirty" and wanted to "give them dirty back," said investigative journalist Mike Boetchetter.

"The main driving force to McVeigh was retribution," said Boetchetter, who was featured in the documentary "An American Bombing." "After he achieved that retribution, he wanted to be the person that jump started the antigovernment movement and then do what he dreamed would be the overthrow of the United States government."

David Cullen, author of "Columbine," wrote that Eric Harris and Dylan Klebold would likely have seen the coverage of Waco and Oklahoma City. Cullen also noted that Harris described wanting to top McVeigh in his journal.

The Columbine attack was originally planned to be on the 19th though the true connection between the horrors was a rage that drove the men to inflict violent terror.

"Most terrorists target symbols of the system they abhor—generally, iconic government buildings. Eric (Harris) followed the same logic. He understood that the cornerstone of his plan was the explosives," Cullen wrote. "Eric (Harris) didn’t have the political agenda of a terrorist, but he had adopted terrorist tactics."

Here's what you need to know about the Waco siege, the Oklahoma City bombing and the Columbine shooting.

Waco siege

• Key Dates: Raid begins at 9:30 a.m. Feb. 28, 1993, siege begins afternoon of Feb. 28, siege ends April 19
• What happened: The Bureau of Alcohol Tobacco and Firearms raided the Mount Carmel compound of the Branch Davidian sect in Waco, Texas. The initial raid, intended to execute a search warrant, left four ATF agents dead and five Branch Davidians. The afternoon following the raid, the Federal Bureau of Investigation led a 51-day siege of the compound. The siege ended when the FBI conducted an assault on the compound leading to a fire that killed 76 Davidians.
• How it is tied to the other events: The siege as well as the Ruby Ridge standoff fed into antigovernment sentiment and was cited by Timothy McVeigh as his cause to commit the Oklahoma City Bombing

Oklahoma City bombing

• Key Date: April 19, 1995
• What happened: Timothy McVeigh detonated a truck filled with nearly 5,000 pounds of explosives in front of the Alfred P. Murrah Federal Building in Oklahoma City. The explosion killed 168 people. McVeigh was convicted on 11 counts of murder and put to death in 2001
• How it is tied to the other events: McVeigh, according to the FBI, visited Waco during the siege and returned to the ruins of the compound in 1994, according to Boetchetter. McVeigh chose the date for his attack to coincide with the anniversary of the siege's end.

Columbine High School shooting

• Key date: April 20, 1999
• What happened: Eric Harris and Dylan Klebold killed 13 people and wounded 24 in a mass shooting at Columbine High School in Columbine, Colorado. The two planted two bombs in the cafeteria of the school that did not detonate. The shooters killed themselves.
• How it is tied to the other events: The shooting was originally planned for April 19, 1999 — the sixth anniversary of the Waco siege ending and the fourth of the Oklahoma City Bombing — according to Cullen. The ability to acquire ammunition pushed the date back to the 20th.