"It's relatively clear that these weren't isolated attacks - even though I can't prove it," he told reporters on Wednesday.
Mehlis, a German prosecutor who headed the UN investigation into al-Hariri's murder, noted that the United Nations last week agreed to provide technical assistance to the Lebanese government to help investigate the killings.
Al-Hariri was killed by a truck bomb in Beirut on 14 February in an attack Mehlis has said involved Syrian intelligence officials and their Lebanese allies.
Several politically motivated killings followed the death of al-Hariri, an opponent of Syria's domination of Lebanon, the most recent being that of newspaper magnate and anti-Syrian legislator Gebran Tueni last week.
Syria has strongly denied any involvement and has also denied dragging its feet over the investigation.
The UN Security Council adopted a resolution last week to extend the investigation into the al-Hariri killing for another six months. But it did not agree to Lebanon's request to broaden the probe into the other killings or to establish a tribunal with an "international character".
Mehlis, who presented a 25-page interim report on al-Hariri's killing to the United Nations last week and is now returning permanently to Berlin, said he doubted six months would be enough to complete the probe.
"It's relatively clear that these weren't isolated attacks - even though I can't prove it"
Outgoing UN investigator
"The next to come will definitely have to be ready for six (months) plus, plus," he said.
In New York, UN Secretary-General Kofi Annan said Mehlis' replacement would be named shortly.
"I am about to name a successor to Detlev Mehlis. But there will be no gap. He will continue until the successor arrives, and I am hopeful they will be able to spend about two weeks together before a successor takes over," he said.
Mehlis declined to speak in detail about the investigation but said the UN team may speak to Farouq al-Shara, the Syrian foreign minister, although he added: "Both sides have to be ready to take part".