Some Ingush officials said Ramzan Kadyrov had arrived unannounced
Chechnya's President Ramzan Kadyrov has vowed a "cruel" revenge on those who tried to kill the leader of Ingushetia, during a visit to the republic.
President Yunus-Bek Yevkurov was wounded in an apparent assassination attempt on Monday.
Mr Kadyrov said on Tuesday he had been authorised by Moscow to take charge of security in Ingushetia - a prospect that alarmed some politicians there.
Mr Yevkurov remains in critical condition in hospital.
He received head and body injuries after a car packed with explosives ploughed into his convoy in the Ingush city of Nazran, in the North Caucasus.
"The federal prosecutors and interior ministry will hold their investigation," Mr Kadyrov said in Ingushetia, on a visit that took some local officials by surprise on Wednesday.
"But we will have an investigation according to mountain traditions and the revenge for Yunus-Bek Yevkurov will be cruel," he said.
"I warn that the terrorists, the inhuman ones, the devils who badly wounded Yevkurov will soon regret it," he said.
Mr Kadyrov, whose own father Akhmad was killed by a bomb in 2004, has brought relative calm to Chechnya, but human rights groups accuse his militias of widespread abuses.
He has been accused of involvement in abduction and torture and a number of apparently political killings, but has always denied it.
On Tuesday Mr Kadyrov said he had been ordered by Russia's President Dmitry Medvedev to run operations in both countries.
The apparent suicide bombing is a worrying development for Moscow
He told Reuters: "He told me to intensify actions... including in Ingushetia. I will personally control the operations... and I am sure in the near future there will be good results."
But Ingush officials insisted they remained in charge of security, and dismissed rumours that the two republics would be merged, as they were in Soviet times.
"All moves to ensure stability in Ingushetia will be co-ordinated by the operation headquarters of Ingushetia," Mr Yevkurov's spokesman said.
And Ruslan Aushev, who ruled Ingushetia from 1992-2001, said: "I think adding additional neighbouring forces would further entangle the situation there. If they want to complicate the situation then this is what to do."
Mr Aushev said he would be willing to take charge of Ingushetia while Mr Yevkurov recovered.