The group mailed numerous letters and cassette recordings to the newspapers, and to radio and television stations during the height of their operations. It is certainly conceivable that all the 1974 'Zodiac' letters were in fact written by the Symbionese Liberation Army, with the January 29th 1974 letter specifically designed to mimic previous Zodiac correspondence. The proof for this militant group's involvement in posing as the Zodiac Killer would come approximately four years later, when the Channel Nine letter was mailed to KHJ-TV in Los Angeles on May 2nd 1978. Arriving just one week after the April 24th 1978 "I am back with you" letter, widely considered a hoax, it may indicate the Symbionese Liberation Army's involvement in both 1978 letters. There was little motivation for the Zodiac Killer to reappear in 1978 and threaten  Daryl Gates  Edward Davis  Pat Boone, and  Eldridge Cleaver, all of which had no relevance to him whatsoever. But the Symbionese Liberation Army certainly had every reason to threaten numbers 1 & 2 on the list - Daryl Gates and Edward Davis.
Her appeals had failed and the Channel Nine letter arrived on May 2nd, threatening to kill both Daryl Gates and Edward Davis. But why were these names top of the letter's hit list?
'While not the first to use specially trained units, the LAPD was the first to form a police tactical unit, and originally created the term, "Special Weapons And Tactics". John Nelson was the LAPD officer who came up with the idea to form a specially trained and equipped unit in the LAPD, intended to respond to and manage critical situations involving shootings while minimizing police casualties. In 1967, Nelson's CO, then-Inspector Daryl F. Gates approved this idea, and he formed a small select group of volunteer officers. This first SWAT unit initially consisted of fifteen teams of four men each, for a total staff of sixty. These officers were given special status and benefits. They were required to attend special monthly training. This unit also served as a security unit for police facilities during times of civil unrest.
In 1971, SWAT personnel were assigned on a full-time basis to Metropolitan Division to respond to continuing action by subversive groups, the rising crime rate and the continuing difficulty of mustering a team response in a timely manner. Metropolitan Division, which had a long-established reputation as the tactical unit of the Department, was reorganized into 6 units: "A", "B", "C", "D", "E", and "K-9" Platoons. The Special Weapons And Tactics Unit was given the designation of "D" Platoon, and at the same time formally adopted the acronym SWAT.
The first significant deployment of LAPD's SWAT unit was on December 9, 1969, in a four-hour confrontation with members of the Black Panthers. The Panthers eventually surrendered, with three Panthers and three officers being injured. By 1974, there was a general acceptance of SWAT as a resource for the city and county of Los Angeles.
On the afternoon of May 17, 1974, elements of a group which called itself the Symbionese Liberation Army (SLA), a group of heavily armed leftists, barricaded themselves in a residence on East 54th Street at Compton Avenue in Los Angeles. In response, more than 400 LAPD officers, Federal Bureau of Investigation (FBI) agents, Los Angeles County Sheriff's Department (LASD) deputies, California Highway Patrol (CHP) officers, and Los Angeles Fire Department (LAFD) firefighters surrounded the residence. Coverage of the siege was broadcast to millions via television and radio and featured in the world press for days after. Negotiations were opened with the barricaded suspects on numerous occasions, both prior to and after the introduction of tear gas. Law enforcement units did not fire until the SLA had fired several volleys of semi-automatic and fully automatic gunfire at them. In spite of the 3,772 rounds fired by the SLA, no civilians or sworn personnel sustained injury from gunfire.
During the gun battle, a fire erupted inside the residence. The cause of the fire is officially unknown, although police sources speculated that an errant round ignited one of the suspects' Molotov cocktails. Others suspect that the repeated use of tear gas grenades, which function by burning chemicals at high temperatures, started the structure fire. All six of the suspects suffered multiple gunshot wounds and perished in the ensuing blaze.
By the time of the SLA shoot-out, SWAT teams had reorganized into six 10-man teams, each team consisting of two five-man units, called elements. An element consisted of an element leader, two assaulters, a scout, and a rear-guard. The normal complement of weapons was a sniper rifle (apparently a .243-caliber bolt-action, judging from the ordnance expended by officers at the shootout), two .223-caliber semi-automatic rifles, and two shotguns. SWAT officers also carried their service revolvers in shoulder holsters. The normal gear issued them included a first aid kit, gloves, and a gas mask. In fact it was a change just to have police armed with semi-automatic rifles, at a time when officers were usually issued six-shot revolvers and shotguns. The encounter with the heavily armed Symbionese Liberation Army, however, sparked a trend towards SWAT teams being issued body armor and fully automatic weapons of various types'. Wikipedia.
'The public became aware of SWAT in May of 1974, when a shoplifting incident at an Inglewood sporting goods store led investigators to a South Los Angeles home on 54th Street. The Symbionese Liberation Army and heiress Patty Hearst, who were barricaded inside, exchanged gunfire with tactical officers for two hours before surrendering. Six SLA members were killed, as thousands of rounds of ammunition was used. The incident, which unfolded on live television, brought home the usefulness of SWAT to Gates, who was the field commander of that incident'. Policemag.com
In an NBC News article reporting his death in 2006, it led with the words "Ed Davis, the tough-talking former Los Angeles police chief who led the department during its shootout with the Symbionese Liberation Army and the arrest of Charles Manson, died Saturday, a family spokesman said. He was 89".
You don't have to search hard on the internet to discover that Daryl Gates and Edward Davis were instrumental in the demise of the Symbionese Liberation Army and the deadly shootout on May 17th 1974 which resulted in the deaths of six of its members. The Zodiac Killer had never crossed swords with either of these two law enforcement officers and had absolutely no reason to want to threaten them with death, as displayed in the Channel Nine letter. On the contrary, an ex-member, close associate or sympathizer of the Symbionese Liberation Army had every reason to want to see these two LAPD servants dead. The author of the Channel Nine letter held them personally responsible for the slaying at 1466 East 54th Street.